02 PO_BT1005_C01_0 TCPIP Basis (2).pdf

DL_BT03_E1 TCPIP Basis

Course Objectives:

Understand the reference models of

TCP\IP and OSI networks

Comprehend the functions and principles

of layers of TCP\IP protocol suite

Master the packet encapsulation and

decapsulation procedures

Grasp IP address types and application

Understand subnet address division

Contents

Chapter 1 ......................................................................... 1

Network Model and TCP/IP Protocol Family .................. 1

Introduction to TCP/IP Protocol Suite ................................. 1

History of OSI Network Model .................................................. 1

Origin of TCP/IP Protocol Family ............................................... 2

Comparison between TCP/IP and OSI Reference Model ............... 3

Packet Encapsulation and Decapsulation ............................. 3

OSI Data Encapsulation Process ............................................... 3

TCP/IP Data Encapsulation Process ........................................... 5

TCP/IP Protocol Family ..................................................... 6

Application Layer Protocols ............................................... 7

Transport Layer Protocols ................................................. 8

Transport Layer Functions ....................................................... 8

Port Numbers ........................................................................ 9

TCP Transport Control Protocol ............................................... 10

User Datagram Protocol UDP ................................................. 16

Network Layer Protocol ................................................... 16

IP Packet Format .................................................................. 17

Protocol Type Field ............................................................... 19

ICMP................................................................................... 20

ARP Working Mechanism ....................................................... 20

RARP Working Mechanism ..................................................... 21

Chapter 2 ....................................................................... 23

Common Network Devices ............................................ 23

HUB ............................................................................. 23

Switch .......................................................................... 24

Router .......................................................................... 25

Routing Switch .............................................................. 26

Comparison between Common Devices ............................. 27

Chapter 3 ....................................................................... 29

IP Address Planning ...................................................... 29

Introduction to IP Addresses ........................................... 29

Types of IP Addresses .......................................................... 29

Reserved IP Address ............................................................ 31

Calculation of Usable Host Addresses ..................................... 32

Addresses with Subnet Division ....................................... 33

Subnet Mask ....................................................................... 34

Examples of Address Calculation ............................................ 35

Variable Length Subnet Mask (VLSM) ............................... 36

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C h a p t e r 1

Network Model and TCP/IP Protocol Family

Knowledge point

Understand TCP/IP and OSI network models

Understand packet encapsulation and decapsulation processes

Master the functions and principles of layers of the TCP/IP protocol suite

Introduction to TCP/IP Protocol Suite

History of OSI Network Model

ISO/IEC (International Organization for

Standardization/International Electrotechnical Commission) is a

voluntary, non-profit and special organization devoted to

international standardization. The OSI model is protocol

international standardization used on various network layers on

the basis of the ISO recommendations. The model is called ISO

OSI open system interconnection reference model, OSI model

for short. The OSI model contains 7 layers. The layers are

classified virtually to realize one determined function for each

layer. The stipulation of the function of each layer is helpful to

clarify the international standard of network protocols. And clear

distinction of the layers is helpful to avoid confusion of functions

of the layers.

With the classification of layers, the information switching issue

of the open system can be resolved through the hierarchic

architecture to the layer of hardware and software modules for


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easy control; each layer can be modified and added with new

functions independently according to its requirement; it is

helpful to perform interconnection between devices from

different manufacturers. And it is helpful for us to study and

understand data communication networks.

Different layer in the OSI reference model provide different

functions. And different layers collaborate to communicate with

each other through standard interfaces.

The application layer provides the network with application

program communication interfaces; the presentation layer

processes data formats and data encryption; the session layer

establishes, maintains and manages sessions; the transport

layer establishes host end-to-end interconnection; the network

layer is responsible for addressing and routing; the data link

layer provides media access and link management; the physical

layer provides bit-stream transport.

The application layer, the presentation layer and the session

layer together are called the upper layer or application layer.

Their functions are usually performed through application

program software; the physical layer, data link layer, network

layer and transport layer together are called data stream layer.

Their functions are mostly performed through cooperation of

hardware and software.

Origin of TCP/IP Protocol Family

TCP/IP can be traced back to a research project of WAN (Wide

Area Network) concerning packet switching (Packet-Switched

Wide-Area Network) by the Advanced Research Projects Agency

(APRA) under the United States Department of Defense (DOD)

in 1969, so the primary network was called ARPANET.

In 1973, TCP (Transport Control Protocol) was officially put into

use; in 1981, IP (Internet Protocol) was put into use; in 1983,

TCP/IP protocols were officially integrated into the UNIX version

of University of California, Berkeley. The operation system of the

Network version met the ardent requirements at that time by universities, institutions and enterprises for network

interconnection. As a result of wide use of the operation system,

which was free of charge then, TCP/IP protocols started to

prevail.

Supported by multiple manufacturers, the TCP/IP technology

soon resulted in many distributed networks. Internet is making

all these individual TCP/IP networks interconnected. The TCP/IP

protocol-based Internet has become a super-large computer

network with the largest scale. It also holds the greatest number

of users and the most abundant resources throughout the world.

The TCP/IP protocols have become an industrial standard as a

matter of fact. The IP networks are becoming the main stream

of computer networks at present and even in the future.

Chapter 1 Network Model and TCP/IP Protocol Family


Comparison between TCP/IP and OSI Reference Model

Like the OSI reference model, TCP (Transfer Control Protocol)/IP

(Internet Protocol) are also developed with different layers; each

layer provides a different function of communication. But there

are fewer layers in the system of the TCP/IP protocolsfour layers from the original seven layers, which are: Application

layer, transport layer, Internet layer and data link layer

respectively from the top down; the presentation layer and

session layer in the OSI model are not available here. Clear

correspondence between the TCP/IP protocol suite and the OSI

model can be seen from the following diagram. The TCP/IP

protocol suite cover all the layers of the OSI model, and the

application layer of TCP/IP protocol suite includes all the upper

protocols of the OSI model.

F I G U R E 1 C O M P AR I S O N B E T W E E N TCP/ IP AN D OSI R E F E R E N C E M O D E L

Packet Encapsulation and Decapsulation

OSI Data Encapsulation Process

Upon receiving data from the upper layer, each layer in the OSI

model will add the control information of this layer into the

header of the data unit. And some layers attach information,



such as checksum, to the trailer of the data unit. This process is

called encapsulation.

The data unit after encapsulation has a different name in

different layers. The protocol data units on the application,

presentation layer and session layer are all called data; on the transport layer, the protocol data unit is called segment; on the network layer, it is called packet; on the data link layer, it is called frame; on the physical layer it is called bits, as shown below.

F I G U R E 2 OSI D AT A EN C A P S U L A T I O N

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data link layer

Physical layer

Application layer

Presentation layer

Session layer

Transport layer

Network layer

Data link layer

Physical layer

When data reaches the receiving end, each layer reads the

related control information and, according to the content of the

information, transfers a data unit to the upper layer. Then the

header and trailer information (if available) of this layer are

removed. This process is called Decapsulation.

This process is implemented layer by layer till the peer end

sends data as required. The required data on the peer

application layer is sent to the related application process of the

local end.

Processes of data encapsulation and decapsulation are described

below with an example of browsing a website by a user.



F I G U R E 3 D AT A EN C AP S U L A T I O N

WWW information

With information of a website inputted by a user, the related

data will be generated by the application layer; the data is

converted through the presentation to an ASCII code that can be

identified by computer; and then the data is transferred to the

transport layer after the related host process is generated by the

session layer. The transport layer treats the above-mentioned

information as the data added with the related port number for

the host to identify the packet. And it assigns the task of the

local computer is to process the information; on the network

layer, the IP address is added, so that the packet is able to

reach the destination host; on the data link layer, the MAC

address is added to convert the packet to bit format, which can

be transported on networks. The packet is received by each

host, which checks the destination MAC address of the packet to

judge whether it is the destination host of the packet; if the MAC

address is found to be inconsistent with its own, it will discard

the packet. If consistent, it will send the packet, with the MAC

information removed, to the network layer to judge the IP

address; finally, the system assigns the related process to

handle the packet, through the destination port number of the

packet,. This is the process of packet decapsulation.

TCP/IP Data Encapsulation Process

Like the data encapsulation of the OSI reference model, the

encapsulation and decapsulation of TCP/IP packets during their

transfer takes place between layers.

On the transmitting side, operations of encapsulation are

performed layer by layer. Each application program sends data

to the transport layer. The transport layer (TCP/UDP) divides the

data into segments of a certain size, and transmits the data to

the network layer with the header of this layer attached. The

packet header of the transport layer contains the port number of



the upper protocol or application program in the packet. For

example, the port number of Telnet is 23. The transport layer

protocol uses a port number to invoke and identify various

application programs of the application layer.

The network layer processes the segment from the transport

layer in certain steps (distinguishes the transport layer protocol,

looks for the address of the next hop, resolves the physical

address of the data link layer, etc.) Then the data is attached

with the packet header of this layer, and converted it to a

packet. The network layer transmits this packet to the data link

layer (Ethernet, frame trunk, PPP, HDLC and so on);

The frame header of this packet layer varies with data link layer

protocols. The data link layer adds header to packet according to

its protocol and then transmits the packets in the form of bits.

On the receiving end, the decapsulation operations are also

performed layer by layer. From the physical layer to the data

link layer, the packet header of each layer is removed layer by

layer. And the data is transferred to the application program for

implementation.

F I G U R E 4 TCP/ IP D AT A EN C AP S U L A T I O N

Application

layer

TCP layer

IP layer

Network

access layer

Subscriber data

TCP segment

IP packet

Frame of

actual physical

network

Application

layer

TCP layer

IP layer

Network

access layer

Subscriber data

TCP segment

IP packet

Frame of

actual physical

network

TCP/IP Protocol Family The TCP/IP protocol suite is composed of different protocols of

different network layers.



F I G U R E 5 TCP/ IP PR O T O C O L F AM I L Y

Network access layer

The network interface layer involves primitive bits that are

transmitted on communication channels. It provides mechanical,

electrical and functional means and processes required for data

transmission. It provides a correct channel for transmitting data

by implementing error detection, error correction and

synchronization. It is also responsible for flow control. The

network layer checks network topology to determine the optimal

route for message transmission, and transfers data. The key

factor is to determine the best route for transmitting data

packets from the source end to the destination end. The main

protocols on the network layer are: IP, ICMP (Internet Control

Message Protocol), IGMP (Internet Group Management Protocol),

ARP (Address Resolution Protocol) and RARP (Reverse Address

Resolution Protocol).

The basic function of the transport layer is to implement

end-to-end communication for the application program between

two hosts. The transport layer receives data from the application

layer, and then divides it into smaller units if necessary. Before

sending it to the network layer, the system ensures that the

data is sent to correct segments of the peer. The main protocols

on the transport layer are: TCP and UDP (User Datagram

Protocol).

The application layer is responsible for processing details of a

specified application program. The application layer displays

received information, transmits user data to the lower layer and

provides network interfaces for application software. The

application layer contains quite a few common application

protocols, such as HTTP (HyperText Transfer Protocol), Telnet

(Remote login), FTP (File Transfer Protocol) and so on.

Application Layer Protocols On the application layer, there are multiple network application

programs developed for user network requirement, such as file

transfer, network management, and even routing. Here are

some common application layer protocols.



FTP (File Transfer Protocol) is an Internet standard for file

transfer. FTP supports file architectures of some text files (such

as ASCII, binary system) and byte-stream-oriented files. FTP

uses TCP of the transport layer to transfer files between terminal

systems that support FTP. FTP provides reliable connection

services, so it is suitable for file transfer on long-distance lines of

low reliability.

TFTP (Trivial File Transfer Protocol) is also used for file transfer.

Because it uses UDP to provide services, it is considered as

unreliable and connectionless. TFTP is usually used for

transferring files within a reliable LAN.

SMTP (Simple Mail Transfer Protocol) supports text mail transfer

through Internet.

Telnet is a standard terminal emulation protocol used by the

client for establishing connection with the remote server.

SNMP (Simple Network Management Protocol) is responsible for

monitoring and maintaining network devices, supports security

management and performance management.

The Ping command is an effective tool that judges whether

network devices are correctly connected.

Similar to the Ping command, the Tracert command is also a

good diagnosis command as it displays the information of each

device the packet passes through.

The DNS (Domain Name System) converts names (for easy

memory) of the network nodes to network addresses.

Transport Layer Protocols

Transport Layer Functions

The transport layer is located between the application layer and

the network layer. It provides end-to-end connection to terminal

hosts, implements flow control (realized by the window

mechanism) and reliability (realized by sequence numbers and

acknowledgment technology), and supports full duplex

transmission. Protocols on the transport layer are: TCP and UDP.

TCP and UDP use the same network layer protocol IP, but they

provide completely different services to the application layer.

Transport Control Protocol TCP: It provides the application

program with reliable connection-oriented communication

services. It is applicable to an application program that requires

response. Currently, many popular application programs use

TCP.

User Datagram Protocol UDP: It provides connectionless

communication, and does not provide assurance for the



reliability of data packets transported. It is applicable to

transporting of small-volume data at one time, while the

reliability is ensured by the application layer.

TCP ensures the reliability of end-to-end data communication in

the following procedure:

1. The TCP entity generates segments by means of dividing an

application program into suitable data blocks with TCP

header.

2. The TCP entity starts the timer immediately after issuing

segments. It will transmit the segments issue again if the

source equipment does not receive an acknowledging

message from the destination equipment after the timer is

cleared.

3. Upon receiving the data, the remote TCP entity returns an

acknowledgement.

4. TCP contains a field of end-to-end checksum, which tests any

change during the data transmission. If the calculation of the

data checksum received by the destination equipment is

incorrect, TCP will discard the segments, and the source

equipment will re-transmit the segments after the timer is

cleared.

5. Since IP provides connectionless and unreliable service, the

TCP data carried in IP packets may be out of sequence. TCP

data can be rearranged. With this function, the destination

equipment rearranges the received data and sends it to the

application program.

6. TCP provides flow control. There is a buffer window on each

end of TCP connection. The destination equipment only

receives the data from the source equipment with its

permission. In this way, it can prevent overflow of the buffer.

7. TCP supports full duplex data transmission.

Port Numbers

TCP and UDP use port numbers of 16bits to express and identify

different application programs in the network. The network layer

protocol IP uses designated protocol numbers (TCP 6 and UDP

17) to express and identify the transport layer protocols.

Each port number between 1 and 1023 represents a type of

service provided by TCP/IP. These port numbers are assigned

and managed by the IANA (Internet Assigned Numbers

Authority). Port numbers smaller than 255 are reserved for

public use; port numbers from 255 to 1023 are assigned to

companies for special use; any port number bigger than 1023 is

called a temporary port number, but not stipulated by the IANA

yet.



Common TCP port numbers are: HTTP 80, FTP 20/21, Telnet 23,

SMTP 25 and DNS 53; common reserved UDP port numbers are:

DNS 53, BootP 67 (server) / 68 (client), TFTP 69 and SNMP 161.

TCP Transport Control Protocol

TCP Packet Format

TCP provides terminal equipment with connection-oriented and

reliable network services. And UDP provides terminal equipment

with connectionless and unreliable packet services. From the

diagram below, to ensure reliability of data transmission, the

TCP packet header offers more options of fields in comparison

with UDP packet header.

F I G U R E 6 TCP P AC K E T FO R M A T

Main fields of a TCP packet header:

Each TCP packet header contains source port numbers (source

port) and destination port numbers (destination port), which are

used to identify and distinguish application processes of the

source equipment and destination equipment. In the TCP/IP

protocol suite, source port numbers and destination port

numbers form a socket together with the source IP address and

destination IP address respectively, which determines a unique

TCP connection.

The sequence number field is used to identify byte streams

transmitted from the TCP source equipment to the destination

equipment. It indicates the first data byte in the packet

segment. If a byte stream is seen as a unidirectional flow

between two application programs, TCP will count each byte by

means of a sequence number. A sequence number is a digit of

32 bits.



Because each byte transmitted is counted, an acknowledgement

number (32 bits) contains the next sequence number which the

receiving end expects to receive. The acknowledgement number

must be the data byte sequence number plus 1 compared with

the number successfully received last time.

TCP flow control is performed by each end connected through

the prompt windows size. Window size is expressed in a data

packet. For example, when Windows size=3, it indicates three

packets can be transmitted once. The window size is specified by

the value in acknowledgement field. The window size is an

adjustable field of 16 bits.

The checksum field is used to check the accuracy of the TCP

packet header and the data part.

TCP Port Number

F I G U R E 7 TCP P O R T N U M B E R

Source port Destinationport

Dest. port=23.Send data

packets to myTELNET port

Host A connects host Z in TELNET mode, where the destination

port number is 23 and the source port number is 1028. There is

no special requirement on the source port number. You only

need to ensure the port number is unique on the local computer.

Generally, you can assign vacant port numbers with value bigger

than 1023. A source port number is also called a temporary port

number. It is because this number functions in a very short

period of time.



F I G U R E 8 U S E O F PO R T NU M B E R I N M U L T I P L E C O N N E C T I O N S

Source port Destinationport

Here is an example of multiple application processes on one host

simultaneously accessing one service. Host A provides two

connections simultaneously in offers Telnet service to access

host Z. Host A use different source port numbers to distinguish

different application processes on the local computer.

An IP address and a port number are used to determine the

unique data communication.

Overview on TCP Serial Number and Acknowledged Number

F I G U R E 9 TCP S E Q U E N C E NU M B E R S AN D AC K N O W L E D G E M E N T N U M B E R S

Sourceport

Destinationport

SerialNo. #

Confirmedserial No. #

Function of a sequence number: It identifies data sequence, so

that the receiver can assemble the data in correct sequence

before transmitting it to the application program. It also

eliminates repeated packets on the network during network

congestion.



Function of an acknowledgement number: The receiver informs

the sender of the segment received successfully. It also informs

the sender the next byte required by the receiver.

: Attention:

An acknowledgement number is the sequence number of the

next data segment required by the receiver. When a data

segment fails to be transmitted, the system will perform

separate acknowledgement and retransmission.

TCP Three-Way Handshake/Connection Established

F I G U R E 10 TCP TH R E E -W A Y H AN D S H AK E / CO N N E C T I O N ES T AB L I S H E D

TCP is a connection-oriented protocol of the transport layer. It

means connection must be completed before data

communication.

The TCP connection process is usually called three-way

handshake, which is as follows:

The requesting end (usually called the client) sends a SYN

segment, indicating the port of a server which the client

wants to connect. And the segment also carries initial

sequence number (ISN). This SYN segment is packet

segment 1.

The server returns a SYN segment (segment 2) containing

the initial sequence number of the server. It sets the

acknowledgement number as the clients ISN plus 1 for acknowledging the clients SYN segment at the same time.

One SYN will occupy a sequence number.

The customer shall set the acknowledgement number as the

servers ISN plus 1 to acknowledge the servers SYN segment (segment 3).

These three segments are used to establish the connection.

The end sending the fist SYN segment performs the functions of

active open, while the other end, which receives this SYN



segment, responses with the next SYN segment to perform the

functions of passive open.

When sending its SYN for establishing connection, one end

selects an initial sequence number (ISN). ISN changes as time

elapses, so each connection will have a different ISN. In RFC

793 [Postel 1981c], an ISN can be seen as a 32-bit counter,

with 1 increased in value every 4 ms. The aim of selecting a

sequence number in this way is to prevent any packet delayed in

the network from retransmission. In this way, misinterpretation

can be avoided between the connected parties.

How to select a sequence number? In 4.4 BSD (and most

Berkeley versions), the sequence number sent is initialized as 1.

This variable increases 64000 every 0.5 second, and returns to 0

every 9.5 hours.

In addition, this variable increases 64000 every time a

connection is established.

TCP Four-Way Handshake/Connection Terminated

F I G U R E 11 TCP FO U R -W AY H AN D S H AK E /C O N N E C TI O N TE R M I N AT E D

Host A Host B

Applicationprogram closed

Applicationprogram closed

ACK of FIN

One TCP connection is duplex (that is, data can be transmitted

in two directions simultaneously), so it must be closed in each

direction individually. Upon accomplishing transmission of data,

one end sends a FIN to terminate the connection in this

direction. Once receiving a FIN, one end must notify the

application layer that the other end has terminated transmission

of data in that direction. So, it takes four processes for TCP to

terminate a connection. This is called Four-Way Handshake.



Basic ConceptWindow Control

F I G U R E 12 TCP W I N D O W CO N T R O L

The window is actually a mechanism of flow control.

When the size of the window is 1, after a data segment is

transmitted, the system waits for an acknowledgement before

transmitting the next data segment. The advantage is that the

correct sequence of data segments received is guaranteed at the

receiving end. And the disadvantage is that the transmission

speed and efficiency are low.

By using a window greater than 1, several packets can be

transmitted simultaneously. When an acknowledgement is

returned, a new data segment can be transmitted. This mode

assures higher transmission efficiency. A well configured sliding

window protocol can keep high efficient packet transmission in

the network. And a relatively throughput can be attained.

The advantage is high transmission speed and high operation

efficiency; the disadvantage is that it may result in incorrect

sequence of data segments at the receiving end. Because

different paths are used during data transmission for TCP/IP, the

sequence of data segments may not be in correct order.



User Datagram Protocol UDP

F I G U R E 13 UDP P AC K E T F O R M AT

In comparison with TCP packet, UDP packet has fewer fields:

Source port number, destination port number, length and

checksum, each filed provides the same functions as the

corresponding filed of TCP packet.

UDP packet does not have reliability assurance, sequence

assurance fields or flow control field, so it is low in reliability. Of

course, you can notice the advantages by using the application

program of the transport layer UDP service. Because of the

fewer control options of the UDP protocol, there is little delay

during data communication, and the high efficiency of data

communication is assured. It is applicable to application

programs that do not have high requirements for reliability, or

applicable to application programs with reliability assurance,

such as DNS, TFTP, SNMP and so on; UDP protocol is also

applicable to networks with reliable transmission links.

Network Layer Protocol The network layer is located between the data link layer and the

transport layer in TCP/IP suite. The network layer receives

packet from the transport layer. Then, it divides the data into

segments of appropriate size, and encapsulates them with an IP

header before sending them to the data link layer. To ensure

successful transfer of packets, the network layer defines the

following protocols:

IP (Internet Protocol): IP collaborates with the route protocol to

find the optimal route for transferring packets to the destination.

Because the IP does not care about the content of packets, it

provides connectionless and unreliable services.

ARP (Address Resolution Protocol): It resolves a given IP

address to an MAC address.



RARP (Reverse Address Resolution Protocol): It resolves an IP

address when the address of the data link layer is provided.

ICMP (Internet Control Message Protocol) defines functions of

network layer control and message transferring.

IP Packet Format

F I G U R E 14 IP P AC K E T FO R M AT

A common IP header is 20-byte long, excluding the IP option

field.

An IP packet contains the following parts:

Version field: It indicates the version number of the IP protocol.

The current protocol version number is 4. The subsequent

version number of the IP protocol is 6.

Header length: it refers to the bytes number of the 32 bit in an

IP header, including other options. Since it is a 4-bit field, the

longest header is 60 bytes. The value of a common IP packet

(without any option) field is 5, that is, 20 bytes in length.

Type of service (TOS) field: It contains a 3-bit priority sub-field.

A 4-bit TOS sub-field and a 1-bit sub-field are unused (they

must be set as 0). The 4-bit TOS represents: minimal time

delay, maximal throughput, highest reliability and minimal

expense respectively. Only one of the 4 bits can be set. If all the

4 bits are 0, it indicates a common service. Currently, TOS is not

supported by most TCP/IP applications, but it is set in the new

versions later than 4.3 BSD Reno. In addition, new route

protocols, such as OSPF and IS-IS, can determine routes

according to the values of these fields.

Total length field: It refers to the length of the whole IP packet,

taking the byte as a unit. With the header length field and total

length field, we can know the initiating location and length of the

data content in the IP packet. Since the field is 16-bit long, the

IP packet can be 65535 bytes at the maximum. Though an IP



packet of 65535 bytes can be transmitted, most link layers will

divide it into segments. The total length field is an indispensable

part of the IP header. Because some data links (such an

Ethernet) need add some data to reach the minimal length as

required. The minimal frame of the Ethernet is 46 bytes, but the

IP data may be shorter. With the total length field, the IP layer

will obtain the content of the IP packet among the 46 bytes

data.

Identification field: It identifies the unique packet that the host

transmits. Usually, whenever a packet is transmitted, its value

will increase by 1. The physical network layer usually limits the

maximal length of the data frame every time it is sent. IP makes

a comparison between lengths of the MTU and the packet, and

divides it into segments if necessary. IP packet can be divided

either on the originating host, or on the intermediate router. A

divided IP packet will be reassembled when it reaches the

destination. Reassembly is accomplished on the IP layer at the

destination end, so that the dividing and reassembling processes

are transparent to the transport layer (TCP and UDP). And the

whole packet is to be re-transmitted even if only a bit of datum

is lost.

A packet fragment may be divided again (possibly for many

packet fragmentations). The data contained in the IP header

provides sufficient information for packet fragmentation and

reassembling.

For every IP packet transmitted from the sending end, its

identification field contains a unique value. The value is copied

into every segment during packet fragmentation. The

identification field uses one of the bits to express more segments; except in the last segment. In each of above segment, this bit must be set to 1.

Fragment offset field: It refers to the position the segment is in

when it starts to take its offset from the original packet. When a

packet is fragmented, the length value of each segment must be

changed to that of the fragmented segment. One bit in the

identification field is called no fragment bit. If this bit is set as 1, the IP will not perform packet fragmentation. During the

network data transmission, if the MTU of the link layer is less

than the packet length, the packet will be discarded and an ICMP

error packet will be transmitted.

TTL (time-to-live): This field specifies the maximal number of

routers a packet can pass through. It specifies the valid duration

(time-to-live) of a packet. The initial value of the TTL (usually 32

or 64) is set by the source host. Once the packet passes through

a router that processes it, the value will be deducted by 1. When

the value of the field is 0, the packet will be discarded, and an

ICMP packet will be transmitted to notify the source host.

Protocol field: This is a field by which we can identify the

protocol transmitting data to the IP.

Header checksum field: This is a checksum code calculated

according to the IP header. It does not calculate any data



following the header. ICMP, IGMP, UDP and TCP all contain a

checksum code in their headers specifying the header and data.

Each IP packet contains 32-bit source IP address and destination

IP address.

The last field is the options, namely, optional information of

variable length in the packet. These options are defined as

follows:

Security and processing restriction (It is usually used in

military field. Refer to RFC 1108 [Kent 1991] for details.)

Recording paths (the IP address of each router is recorded);

Time stamp (The IP address and routing time of each router

is recorded);

Loose routing of source sites (providing a series of IP

addresses a packet pass through);

Strict routing of source sites (similar to loose routing of

source sites, but it requires that a packet can pass through

these addresses only. That is, the routing is fixed).

These options are seldom used, which are not supported by all

hosts and routers. The option field always takes 32 bits as the

boundary. Filling bytes with value of 0 can be inserted if

necessary. In this way, the IP header is always an integer

multiple of 32 bits.

Lastly, data of the upper layer, such as data segments of TCP or

UDP.

Protocol Type Field

F I G U R E 15 PR O T O C O L TY P E F I E L D

Transport layer

Network end

Protocol number

TCP, UDP, ICMP, IGMP and some other protocols all use the IP to

transmit data. A flag must be added into the IP header

generated to identify the type of the data. For this purpose, an

8-bit long value is stored in the IP header. This value is called

Protocol domain.



Where, 1 represents ICMP, 2 represents IGMP, 6 represents TCP

and 17 represents UDP.

ICMP

ICMP is a protocol that integrates error report and control. It can

be used on all TCP/IP hosts. ICMP messages are encapsulated in

an IP packet. ICMP is often considered as a component of the IP

layer. ICMP transfers error packets and other important

information. ICMP packets are usually used by protocols of the

IP layer or upper layers (TCP or UDP). Some ICMP packets are

used to return error packet to the user process.

The common ping command uses the ICMP. The word ping is originated from locating operations by sonar. The objective is to

test whether another host is reachable. This program sends a

request packet for ICMP response to the host, and waits for the

response from the ICMP. Generally, if we cannot Ping a host, we

cannot use Telnet or FTP to connect the host either. On the

contrary, if we cannot use Telnet to connect a host, we can

usually use the Ping program to locate the problem. The Ping

program can also test how long it takes to reach and return from

the host. In this way, we can figure out how far the host is away from us.

However, with stronger consciousness of Internet security, more

and more routers and firewalls provide access control. The

above assertion may not function sometimes. We cannot only

confirm a reachable host with its reachable IP layer. We must

also take protocol and port number used into consideration.

ARP Working Mechanism

F I G U R E 16 ARP W O R K I N G M E C H AN I S M

I need thephysical addressof a host whose

IP address is176.16.3.2.

I hear the broadcastpacket. The messageis for me. Here is my

physical address.

The data link layer protocols, such as Ethernet or token ring

network, have their own addressing mechanism (usually a 48-bit



address). This is a rule any network layer that uses data links

must obey. When a host transmits an Ethernet data frame to

another host on the same LAN, it determines the destination

interface according to the 48-bit Ethernet address. The

equipment driver never checks the destination IP address in an

IP packet.

The ARP must provide correspondence between an IP address

and an MAC address.

ARP process: The ARP sends an Ethernet data frame (called ARP

request) to each host on the Ethernet. This process is called

broadcast. The ARP request data frame contains the IP address

of the destination host, which means If you are the owner of this IP address, please reply your hardware address.

All the hosts in the same LAN must receive and process the ARP

broadcast. After receiving the broadcast packets, the ARP layer

of the destination host will judge, according to the destination IP

address, that the originating end is querying its MAC address.

So, it sends a unicast ARP response, which contains the IP

address and the corresponding hardware address. Upon

receiving this ARP response, the originating end can obtain the

MAC address of the receiving end.

The key to the highly efficient ARP operation is that each host

provides an ARP cache. This cache stores the recent mapping

record between the IP address and the hardware address. When

a host wants to query the correspondence between the IP

address and the MAC address, it must look for it in the local ARP

cache table. It will resort to ARP broadcast only when it cannot

be found.

: Knowledge point

ARP request is in broadcast mode while ARP response is in

unicast mode.

RARP Working Mechanism

F I G U R E 17 R ARP W O R K I N G M E C H AN I S M

What is myIP address?

I hear thebroadcast packet.Your IP addressis 172.16.3.25



For a system with the local disk, the IP address is usually read

from the configuration file on the local disk. However, we need

to use other methods to get the IP address of a diskless

workstation or a host configured with dynamical IP address.

RARP process: The host reads a unique hardware address from

the interface card. Then it sends an RARP request (data

broadcast on the network), asking a certain host (such as the

DHCP server or BOOTP server) to assign an IP address the host

system in response.

Upon receiving the RARP request, the DHCP server or BOOTP

server assigns configuration information (IP address and so on)

to it and returns an RARP response to the source host.


C h a p t e r 2

Common Network Devices

Knowledge point

Understand the functions and working principles of common network devices in an IP network

HUB

F I G U R E 18 HU B

Located on the physical layer, a HUB provides following

functions: signal regeneration and amplification, and noise

elimination. A network that connected by a HUB is a star

topology in physical, but it is a bus topology in logical. All

workstations connected by a HUB share the same transmission

media, so all the devices are located in the same broadcast

domain, and share the same bandwidth.

: Attention:

For a 10M HUB, 10M is the physical bandwidth. The effective

bandwidth shared by all the hosts connected to this HUB is less

than 10M due to protocol overhead incurred by collision and

other events in the Ethernet.

Ethernet uses a CSMA/CD (Carrier Sense Multiple Access with

Collision Detection) mechanism. When more terminals are

connected in the network, there will be more collisions. If too

many hosts are connect in a collision domain, large number of



collisions will occur. More occupied bandwidth causes lower

network performance, or even network breakdown.

: Knowledge point

Collision is not a fault in an Ethernet. The collision mechanism

can be seen as a flow control mode used for the Ethernet.

However, if there are abnormal collisions in the Ethernet, faults

will occur to the network.

Switch A HUB only provides signal regeneration and amplification. Using

Hubs in the network, all the devices share a transport medium,

and perform data exchange in CSMA/CD mode. All workstations

in the HUB network are configured in the same collision domain

and the same broadcast domain.

The layer-2 switch is a data link layer device.. It performs

switching by reading the MAC address information in a packet. It

isolates the collision domain and works on the data link layer.

So, each port of the switch is an individual collision domain.

There is an address table in the switch. The address table shows

the mapping between the MAC address and the switch port.

When receiving a packet from a port, the switch first reads the

source MAC address in the packet header. Then it can obtain the

port connected to the machine with this source MAC. With the

destination MAC address in the packet header, the switch

searches the related port from the address table. If a port

corresponding to the destination MAC address is available in the

table, it copies the packet directly onto the port. If no

corresponding port is found in the table, it will broadcast the

packet to all ports. When the switch receives the response of the

destination machine, it can obtain the port corresponding to the

destination MAC address. In this way, the switch will not have to

broadcast to all the ports in transmitting data the next time.

Above section describes how layer-2 switch establishes and

maintains its own address table. Layer-2 switch usually

possesses a broad switching bus bandwidth to exchange data

with multiple ports simultaneously. Suppose the layer-2 switch

provides N ports, and each has a bandwidth of M. If its switching

bus bandwidth is greater than NM. the switch can enable

wire-speed switching. Layer-2 switch imposes no limit to

broadcast packets, and it copies broadcast packets to all the

ports.

Layer-2 switch can transfer packets in relatively high rate due to

an ASIC (Application Specific Integrated Circuit) chip especially

for packets transfer.

Chapter 2 Common Network Devices


Router A router operates on the third layer of OSI model, namely, the

network layer.

The routing table inside a router directs packet routing. When

the router receives a packet from a certain port, it removes the

link layer packet header (Packet disassembly). Then, it queries

the routing table with the destination IP address carried in the

packet. If the address of the next intermediate destination is

determined, the packet header of the link layer will be added

(Packet assembly) before the packet is transferred. If not, the

router will send a response message to the source address, and

discard this packet.

F I G U R E 19 RO U T E R O P E R A T I N G PR O C E S S

Route Table Route TableNetworks Interfaces Networks Interfaces

The routing technology looks somewhat similar to layer-2

switch, but there is a difference: Switching takes place on the

second layer of OSI model (the data link layer), whereas routing

takes place on the third layer. Therefore, different control

information shall be used for routing and switching during data

transfer. Their own functions are provided in different modes.

The routing technology actually involves two basic activities:

determining the optimal path and transferring packets. Packet

transfer is simple and direct, while packet routing is relatively

complex. The routing algorithm writes various kinds of

information into the routing table. The router will choose the

optimal path for packet transmission according to the

destination. The router sends the packet to the next router

through the optimal path that can reach the destination. Upon

receiving the packet, the next router will, according to the

destination address, transfer the packet to the subsequent

router through an appropriate path. In this way, the packet can

be sent to the destination through various intermediate routers.

Router can communicate with each other, can maintain and

update their own routing table through exchanging different

types of messages. Route update message is composed of



partial or all routing table information. By way of analyzing route

update messages from other routers, a router can obtain the

topology of the entire network. Link state broadcast is another

kind of common message that is transferred between routers.

This message is used to timely inform other routers of the

senders link state.

Routing Switch Routing switch is also called the layer-3 switch. It is a layer-2

switch providing the layer-3 routing functions. However, it is

organic combination of the two, instead of simply overlaying the

hardware and software of the router equipment onto the LAN

switch.

In terms of the hardware, the interface modules of the layer-2

switch exchange data by way of the high-rate backplane/bus (as

high as scores of Gbit/s). In the layer-3 switch, the layer-3 route

hardware modules related with the router are also inserted on

the high-rate backplane/bus. This mode allows high-speed data

exchange between the route modules and other modules, hence

eliminating the transmission rate limit of the traditional external

router interfaces.

In terms of software, the layer-3 switch also regulates the

traditional router software in the following procedure. For packet

transfer: for example, IP/IPX packet is transferred in high speed

through hardware configuration. The layer-3 routing software

can be used for: route information update, routing table

maintenance, route calculation, and route determination. For

example, they can be enabled through optimized and high

efficient software.

Suppose two machines (using IP) communicate with each other

through the layer-3 switch. Machine A acquires the destination

IP address when starting transmission. However, it does not

obtain the MAC address, which is required for transmitting on a

LAN. Then it uses address resolution protocol (ARP) to obtain the

destination MAC address. Machine A makes a comparison

between its own IP address and the destination IP address. It

checks whether the destination machine is located in the same

subnet with the network address converted from its subnet

mask. If the destination machine B and machine A are located in

the same subnet, machine A broadcasts an ARP request to

machine B. Then machine B returns its MAC address. Upon

getting the MAC address of B, machine A caches the address,

and uses the MAC address to perform data encapsulation. The

layer-2 switching module queries the MAC address table and

determines to transfer the packet to the destination port.

If the two machines are not in the same subnet, machine A

needs to communicate with the destination machine C. A must

send an ARP packet to the Default gateway, whose IP address is already configured in the system software. This IP address

Chapter 2 Common Network Devices


actually corresponds to the layer-3 witching module of the

lsyer-3 switch. Therefore, when machine A broadcasts an ARP

request to the IP address of the Default gateway, if the layer-3 switching module has acquired the MAC address of the

destination machine C, it will reply the MAC address of machine

C to machine A. Otherwise, the layer-3 switching module will

broadcast an ARP request to destination machine C according to

the route information. When getting the ARP request, the

destination machine C returns its MAC address to the layer-3

switching module. The layer-3 switching module stores the

address and replies it to machine A. For subsequent packet

exchange between A and C, the MAC address of destination

machine C will be used for data encapsulation. The layer-2

switch is responsible for data forwarding, thus ensuring

high-speed exchange of information. This is the so-called

Routing for once and switching for multi-times.

The layer-3 switching presents the following features:

Organic hardware combination allows higher speed of data

switching.

Optimized routing software enables higher efficiency of routing

process.

Most of the data transfer process is processed by the layer-2

switch unless otherwise specified by compulsory route process;

In case of interconnection of multiple subnets, only logical

interconnections are made with the layer-3 switching module,

instead of adding ports for external routes in the traditional way.

This helps to protect user investment.

Comparison between Common Devices Normally, the layer-2 switch is used in a small-size LAN, with 20

to 30 machines. In such network environment, broadcast packet

is not a very big issue. The layer-2 switch features quick

switching functions, multiple access ports and low price. This can

be a complete solution for small-scale network users. In this

type of the network environment, it is unnecessary to adopt the

routing function, which involves higher deployment difficulty and

higher cost in management. The layer-3 switch is not required

either.

The layer-3 switch is designed for the IP with simple type

interfaces. It provides powerful layer-2 processing capability, so

it is applicable to a large-size LAN. To reduce the risk of a

broadcast storm, a large-size LAN must be divided into several

small-size LANs, namely, small network segments. This will

arouse communication between these different network

segments, which the layer-2 switch alone is unable to support. If



only routers are used in the network, the network scale and

access rate are limited, due to the limited number of router

ports and low rate of routing. In this case, the layer-3 switch is

the most appropriate solution because it integrates the layer-2

switching and routing technology.

Routers provide multiple types of ports to support multiple

layer-3 protocols with its powerful routing capability. They are

applicable to interconnection between large-scale networks.

Many layer-3 switches or even layer-2 switches provide ports for

interconnection between heterogeneous networks. However,

large-scale networks usually do not provide many

interconnection ports. Instead of quick switching between ports,

the main function of the router is to select the optimal path. The

routes are also able: to share the load, to perform link backup

and, the most important, to conduct information exchange with

other networks.

For large-scale network construction, it is impossible to use the

layer-2 switch. However, we are required to use the layer-3

switch based on specific conditions. The main factors here

include: network traffic amount, requirement on response rate,

and investment budget. The most important objective of the

layer-3 switch is to accelerate data exchange within a large-size

LAN. Its routing function integrated is also to serve this

objective, which is not as powerful as that of a professional

router of the same class. In case of large network flow, if the

layer-3 switch serves both for intra-network switching and

inter-network routing, its load will be inevitably heavy. Its

response rate is surely affected. In this case, to guarantee high

response rate of the layer-3 switch, we can employ routers to

share the routing processing of the layer-3 switch. It will be

satisfactory collaboration for the layer-3 switch to act for

intra-network switching, while the routers do the routing works,

so as to bring the superiority of different devices to full play. Of

course, if the budget is limited, it will also be a good choice with

the layer-3 switch also serving for interconnection between

networks.


C h a p t e r 3

IP Address Planning

Knowledge point

Understand IP address types and application

Understand classification of subnets

Introduction to IP Addresses The specifications of the Internet Protocol (IP) were set up by

RFC791 in 1982. Some contents of the specifications stipulate

the structure of IP addresses. The structure provides each host

and router interface with 32-bit binary logical addresses,

including the network part and the host part.

For easy writing and remembering, one IP address is usually

expressed by 4 decimal digits within 0~255, with a period

separating each adjacent two digits. Each of these decimal digits

represents 8 bits of the 32-bit address, namely the so-called

octet. This is called dotted decimal notation.

F I G U R E 20 IP AD D R E S S E S

Types of IP Addresses

The address types are classified according to network scale,

shown as following allows:

Class A: super-large networks



Class B: medium-size networks of limited number

Class C: small-size network of large number

Special class: Class D (for multi-point transmission) and Class E,

usually for test and research purpose

F I G U R E 21 IP AD D R E S S TY P E S

Types of IP addresses can be determined by way of checking the

first octet in the address (the most important). The highest bit

value determines the type of address. The bit format also

defines the decimal value range of the octet related with each

address type.

Class A:

For class A addresses, 8 bits are assigned to the network

address and the other 24 bits are assigned to the host address.

If the most significant bit of the first octet is 0, the address is a

class A address.

This corresponds to the possible octet of 0~127. Among these

addresses, 0 and 127 are reserved, so the actual value range is

1~126. Among type A addresses, only 126 networks can be

used. Since only 8 bits are reserved for the network address, so

the first bit must be 0. However, the digits for a host can be of

24 bits, so, each network can supports up to 16,777,214 hosts.

Class B:

Of class B addresses, 16 bits are assigned to the network

address and the other 16 bits are assigned to the host address.

A type B address can be identified by means of the first two bits

(set to 10) of the first octet. This corresponds to values of

128~191. Since the first two bits have been pre-defined,

actually 14 bits are reserved for the network address. Therefore,

the possible combination generates 16,384 networks, whereas

each network supports 65,534 hosts.

Class C:

Chapter 3 IP Address Planning


Of class C addresses, 24 bits are assigned to the network

address and the other 8 bits are reserved for the host address.

In class C addressthe first three bits of the first octet is 110. This corresponds to decimal digits of 192~223. Among class C

addresses, only the last octet is used for the host addresses.

This imposes a limit that each network can have 254 hosts at

the maximum. Now that there are 21 bits that can be used as a

network number (3 bits have been preset as 110), there can be

2,097,152 possible networks.

Class D:

A class D address starts from 1110. This means that the octet is

within 224~239. These addresses are not used as standard IP

addresses. On the contrary, class D addresses refer to a group

of hosts, which are registered as multi-point transmission group

members. The assignment list of multi-point transmission group

is similar to that of emails. You can use names in an assignment

list to send a message to a user group. You are also able to send

data to some hosts by way of multi-point transmission

addresses. Multi-point transmission needs be configured with

special routes. It will not be transferred by default.

Class E:

If the first four bits of the first octet are set as 1111, the address

is a class E address. These addresses are within the range of

240~254; addresses of this type are not used as the common IP

addresses. Addresses of this type are sometimes used in

laboratories or for research.

We focus on types A, B and C in our discussion, for they are

used for conventional IP addressing.

Reserved IP Address

An IP address is used to identify a unique network device.

However, not all IP addresses can be used. Some special IP

addresses are used for various purposes, instead of identifying

network devices.

An IP addresses with 0 exclusively for the whole host bits is called network address. A network address is used for identifying

a network segment. For example, class A address 1.0.0.0,

private addresses 10.0.0.0, and 192.168.1.0 are network

addresses.

An IP addresses with 1 exclusively for the whole host bits is called network segment broadcast address. A network segment

broadcast address is used to identify all the hosts of a network,

for example, 10.255.255.255, 192.168.1.255, and so on. A

router can transfer broadcast packets on network segments as

10.0.0.0 or 192.168.1.0. A broadcast address is used for

transmitting packets to all nodes of the local network segment.



An IP addresses with 127 for the network part, such as 127.0.0.1, is usually for loop test.

An IP addresses with the value 0 configured for all bits, such as 0.0.0.0, represents all the hosts. On a router, address 0.0.0.0

is used for designating the default route.

An IP addresses with the value 1 configured for al bits, such as 255.255.255.255, is also a broadcast address. The address

255.255.255.255 represents all the hosts, which is used for

transmitting packets to all nodes of the network. Broadcast like

this cannot be transferred by a router.

Calculation of Usable Host Addresses

As mentioned above, there may be some IP addresses in each

network segment that cannot be used as IP addresses for hosts.

Now, lets calculate the available IP addresses.

F I G U R E 22 C AL C U L A T I O N O F NU M B E R O F AV AI L AB L E HO S T AD D R E S S E S

In class B network segment 172.16.0.0, there are 16 host bits,

so there can be 216 IP addresses accordingly. With one network

address 172.16.0.0 and one broadcast address 172.16.255.255

deducted (they cannot identify a host), there will be 216-2

addresses available for hosts. In type C network segment

1192.168.1.0, there are 8 host bits, so there can be 28(256) IP

addresses; with one network address 192.168.1.0 and one

broadcast address 192.168.1.255 deducted, there will be 254

addresses available for hosts. We can calculate the addresses

available for hosts in each network segment with following

method: If there are n bits for hosts in the network segment,

the number of addresses available for hosts will be: 2n-2.



A network layer device (such as a router) uses a network

address to represent the hosts in the network segment, thus

greatly reducing entries of the routing table of the router.

Addresses with Subnet Division Any IP address organization without subnet will be considered as

a single network. It is not necessary to know its internal

architecture. For instance, all routes to address 172.16 .X.X are

considered as in the same direction, so the third and fourth

octets of the address will not be taken into consideration. A plan

like this can have fewer entries in the routing table.

F I G U R E 23 AD D R E S S I N G W I TH O U T SU B N E T

However, this plan is unable to distinguish different subnet

segments in a large network. In this case, all the hosts in the

network receives broadcast in the large network. Therefore, it

will reduce the network performance, and hinder the network

management.

For example, a class B network can accommodate 65000 hosts,

but it is too difficult to manage so many hosts simultaneously.

So we need to divide such a network into different segments. In

this way, we can manage the subnet according to network

segments. Usually, host bits can be divided into subnet bits and

host bits.



F I G U R E 24 AD D R E S S I N G W I TH SU B N E T S

In this example, the subnet bits occupy the 8 bits of the third

segment. Compared with the previous example, the original

class B network is divided into 256 subnets, and the number of

hosts each subnet can accommodate is reduced to 254.

When different subnets are divided, different logical networks

are created accordingly. The routers are responsible for

communication between these different networks. That is, an

original large broadcast domain is divided into multiple smaller

broadcast domains.

A network device uses a subnet mask to identify network bits,

subnet bits and host bits. The network device can distinguish the

destination address of an IP packet, according to the IP address

and subnet mask configured. The network device can distinguish

whether the destination address of an IP packet and its address

are located in the same subnet, or in the network of same type

but in different subnets, or in networks of different types.

Subnet Mask

An IP address without the related subnet mask is of no

significance.

A subnet mask defines how many bits from the 32 bits of an IP

address are used as the network bits, or as bits for the network

and its related subnet bits.



F I G U R E 25 SU B N E T M AS K

Network bits

IP address

Host bits

Default mask

It can also be "/16", where 16 is the digits for the mask

8-bit subnetmask

It can also be "/24", where 24 is the bits for the mask

Network bits Host bits

Network bits Host bitsSubnet bits

The binary bits in the subnet mask can be used as a filter, which

calculates the network address by identifying the part of the IP

address of the network address. The process of this task is

called Bitwise AND.

Bitwise AND is a logical operation, which performs calculation of each bit of the address and the corresponding mask bit.

To divide a subnet is actually to borrow the host bits in the

original address to be used as the subnet bits. It is currently

stipulated that bits shall be borrowed from the left to the right in

succession, that is, the 1 and 0 in the subnet mask shall be

consecutive.

Examples of Address Calculation

F I G U R E 26 EX AM P L E S O F AD D R E S S C AL C U L AT I O N

Above are examples of address calculation:



For given IP address and subnet mask, the address calculation

involves: the address, the broadcast address and the available

IP address range of the subnet where the IP address is located.

Convert the IP address to one presented in the binary

system.

Also convert the subnet mask to one presented in the binary

system.

Draw a vertical line between 1 and 0 of the subnet mask.

Bits on the left side of the line are for the network (including

subnet), and bits on the right are for the host.

Set all the host bits as 0. The network bits are the network

address of the subnet.

Set all the host bits as 1. The network bits are the broadcast

address of the subnet.

The available IP addresses range from the network address

to the broadcast address of the subnet.

Complete the above three network addresses.

Finally, convert them to the decimal numbers.

Variable Length Subnet Mask (VLSM) When defining the subnet mask, suppose that the mask will be

used in a unified way throughout the network. This setting

causes waste of many host addresses.

F I G U R E 27 EX AM P L E O F V AR I AB L E LE N G T H SU B N E T MAS K

For instance, a subnet connects 2 routers by way of serial

interfaces. On the subnet, there are only 2 hosts, each

connecting a port. We have assigned the whole address of the

subnet to the two interfaces, thus many IP addresses will be

wasted.



If we use one of the subnets, we can divide it into level-2

subnets. In this way, we can effectively establish subnets of subnets and reserve other subnets. Then we will use IP addresses to the maximal extent. The concept of establishing

subnets of subnets is the foundation of VLSM.

To use the VLSM, we usually define a basic subnet mask, which

will be used for dividing the level-1 subnet. With this subnet

mask, a level-2 mask will be used for dividing one or more

level-1 subnets.

The VLSM can be identified by a new route protocol only, such

as BGP, OSPF or RIPv2.

: Attention:

VLSM is supported by static routing.

02 PO_BT1005_C01_0 TCPIP Basis (2).pdf

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