Chapter 5

Network Layer

João José

jjose@ualg.pt

http://w3.ualg.pt/~jjose/cisco/

Based on:

Graziani, R. (2008) CIS 81 Networking Fundamentals

Cisco CCNA 1 Exploration - Networking

Network Layer Overview

Network Layer

IPv4

Application

Header + data

IP Header

0 15 16 31

4-bit

Version

4-bit

Header

Length

8-bit Type Of

Service

(TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit

Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

Network Layer

The Network layer (Layer 3) provides services to exchange the data over

the network between identified end devices.

Layer 3 uses four basic processes:

Addressing

Encapsulation

Routing

Decapsulation

0 15 16 31

4-bit

Version

4-bit

Header

Length

8-bit Type Of

Service

(TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit

Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

Addressing

0 15 16 31

4-bit

Version

4-bit

Header

Length

8-bit Type Of

Service

(TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit

Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

172.16.3.10192.168.100.99 Source IP = 192.168.100.99

Destination IP = 172.16.3.10

Destination IP = 192.168.100.99

Source IP = 172.16.3.10

Source IP Address

Destination IP Address

More later!

DataHTTP

Header

TCP

Header

IP

Header

Data Link

Header

Data Link

Trailer

IP PacketData Link

Header

Data Link

TrailerIP PacketData Link

Header

Data Link

Trailer

IP PacketData Link

Header

Data Link

TrailerIP PacketData Link

Header

Data Link

Trailer

IP PacketData Link

Header

Data Link

TrailerIP PacketData Link

Header

Data Link

Trailer

Encapsulation and Decapsulation

DataHTTP

Header

TCP

Header

IP

Header

Data Link

Header

Data Link

Trailer

Decapsulation

Destination

Arrival packet processed at Layer 3.

Destination address examined.

If the address is correct segment is passed up to the appropriate service at Transport layer.

Application

Header + data

Routing

Routers examine Layer 3 Destination IP addresses to forward packets.

Search their routing tables for a match with a network address.

Send the packet on to the next-hop router.

Last router in path forwards the router to the host matching the Destination IP address of the packet.

0 15 16 31

4-bit

Version

4-bit

Header

Length

8-bit Type Of

Service

(TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit

Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

172.16.3.10192.168.100.99

Source IP = 192.168.100.99

Destination IP = 172.16.3.10

Network Layer Protocols

The Internet Protocol (IPv4 and IPv6) is the most widely-used Layer 3 data carrying protocol and will be the focus of this course.

IPv4 basic characteristics:

Connectionless

IP packets are sent without notifying the end host that they are coming.

TCP: A connection-oriented protocol does requires a connection to be

established prior to sending TCP segments.

UDP: A connectionless protocol does not require a session to be

established.

Best Effort Service (unreliable)

The mission of Layer 3 is to transport the packets between the hosts while placing as little burden on the network as possible.

Speed over reliability

Layer 3 is not concerned with or even aware of the type of datacontained inside of a packet.

This responsibility is the role of the upper layers as required.

Unreliable: IP does not have the capability or responsibility to manage, and recover from, undelivered or corrupt packets.

TCP’s responsibility at the end-to-end hosts

Responsibility of the OSI Data Link layer to take an IP packet and prepare it for transmission over the communications medium.

Transport of IP packets is not limited to any particular medium.

In some cases a router will need to split up a packet when forwarding it from one media to a media with a smaller MTU.

fragmenting the packet or fragmentation.

Media Independent

IP Header

IP Destination Address

32-bit binary value that represents the packet destination

Network layer host address.

IP Source Address

32-bit binary value that represents the packet source Network

layer host address.

When a packet is first generated a value is entered into the TTL field.

Originally, the TTL field was the number of seconds, but this was difficult to

implement and rarely supported.

Now, the TTL is now set to a specific value which is then decremented by each

router.

IP’s TTL – Time To Live field

IP’s TTL – Time To Live field

If the router decrements the TTL field to 0, it will then drop the packet (unless

the packet is destined specifically for the router, I.e. ping, telnet, etc.).

Common operating system TTL values are:

UNIX: 255

Linux: 64 or 255 depending upon vendor and version

Microsoft Windows 95: 32

Other Microsoft Windows operating systems: 128

Decrement by 1, if 0

drop the packet.

The idea behind the TTL field is that IP packets can not travel around the

Internet forever, from router to router.

Eventually, the packet’s TTL which reach 0 and be dropped by the router,

even if there is a routing loop somewhere in the network.

IP’s TTL – Time To Live field

Decrement by 1, if 0

drop the packet.

IP’s Protocol Field

Protocol field enables the Network layer to pass the data to the

appropriate upper-layer protocol.

Example values are:

01 ICMP

06 TCP

17 UDP

IP Fragmentation

A router may have to fragment a packet when forwarding it from one

medium to another medium that has a smaller MTU.

If Don’t Fragment flag set, it will not fragment packet, but discard it.

Fragment Offset field and More Fragments flag is used to reconstruct the

packet at the destination host.

Data = 1480 bytesIP

Data = 500IP

Data = 480IP

IP Packet

Fragments

Original IP

Packet

IP Header = 20 bytes

Data = 500IP

Data = 500L2 L2

IP Fragmentation

When fragmentation occurs, it does not get reconstructed until it

reaches the host.

This takes processing time.

Fragment Offset field identifies the order

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP PacketIP Packet

Network link with

larger MTU

Network link with

smaller MTU

Network link with

larger MTU

Other IPv4 fields

Version - Contains the IP version number (4)

Header Length (IHL) - Specifies the size of the packet header.

Packet Length - This field gives the entire packet size, including header and data, in bytes.

Identification - This field is primarily used for uniquely identifying fragments of an original IP packet

Header Checksum - The checksum field is used for error checking the packet header.

Options - There is provision for additional fields in the IPv4 header to provide other services but these are rarely used.

Host and Network Addresses

IP Addresses – First look

Host IP addresses are IP addresses assigned to end devices such as:

Client computers

Server computers

Printers

Router interfaces

Note: the /16 refers to the subnet mask, which will be discussed later.

Note: Intermediary devices such as a switch may have an IP address to allow the network administrator to Telnet to the device for remote management.

Kiwi Airliners - Network Address 172.16.0.0/16

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16

172.16.20.77/16

172.16.20.96/16

172.16.20.103/16

172.16.30.39/16

172.16.30.10/16

172.16.30.111/16

172.16.40.123/16

172.16.40.51/16

172.16.40.29/16

172.16.1.1/16

IP Addresses – First look

Default Gateway

A router which is used to forward packets out of the network.

This is a host IP address on the router.

The default gateway IP address is typically a host IP address which is on the same network as the host itself.

The host only has to be aware of:

Its own network address

Default gateway IP address to reach all devices outside its own network

Network Address 172.16.0.0

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16

172.16.1.1/16

ISP

Internet

Network Address

192.168.1.0/30

192.168.1.2/30

192.168.1.1/30

IP Addresses – First look

All hosts in the same network will typically have the same default gateway IP address.

Network Address 172.16.0.0

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16

172.16.1.1/16

ISP

Internet

Network Address

192.168.1.0/30

Gateway: 172.16.1.1

Gateway: 172.16.1.1

Gateway: 172.16.1.1

192.168.1.2/30

192.168.1.1/30

Confirming IP Address, Default Gateway

C:\> ipconfig

Windows IP Configuration

Ethernet adapter Local Area Connection:

Connection-specific DNS Suffix . :

IP Address. . . . . . . . . . . . : 172.16.10.100

Subnet Mask . . . . . . . . . . . : 255.255.0.0

Default Gateway . . . . . . . . . : 172.16.1.1

Root# ifconfig

eth0 Link encap:Ethernet HWaddr 00:0F:20:CF:8B:42

inet addr:172.16.1.100 Bcast:172.16.255.255 Mask:255.255.0.0

UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1

RX packets:2472694671 errors:1 dropped:0 overruns:0 frame:0

TX packets:44641779 errors:0 dropped:0 overruns:0 carrier:0

collisions:0 txqueuelen:1000

RX bytes:1761467179 (1679.8 Mb) TX bytes:2870928587 (2737.9 Mb)

Interrupt:28

Linux: netstat –rn for default gateway information.

Subnets

Networks can be subdivided into subnets.

This provides for several benefits which we will discuss later.

Networks can be grouped based on factors that include:

Geographic location, Purpose, Ownership

Kiwi Airliners - Network Address 172.16.0.0/16

172.16.10.100/24

172.16.10.55/24

172.16.10.3/24

172.16.20.77/24

172.16.20.96/24

172.16.20.103/24

172.16.30.39/24

172.16.30.10/24

172.16.30.111/24

172.16.40.123/24

172.16.40.51/24

172.16.40.29/24

172.16.1.1/24

172.16.10.1/24 172.16.20.1/24 172.16.30.1/24 172.16.40.1/24

172.16.10.0/24 172.16.20.0/24 172.16.30.0/24 172.16.40.0/24

A Quick Look at Routing

Routing – First Look

Routers know about:

Directly connected networks (C):

Network addresses of its interfaces

Remote networks

Static routes

Dynamic Routing Protocol (R = RIP)

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routers know about:

Directly connected networks (C):

Network addresses of its interfaces

When a router is configured with the IP address/mask on an interface the

router knows that it has an interface which is part of that network.

This is just like a host that is configured with an IP address/mask. (coming)

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

Routers learn about remote networks using:

Static routes

Dynamic Routing Protocol (R = RIP)

Routes in a routing table have three main features:

Destination network

Next-hop

Metric

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

Static routes

Manually entered by the administrator

Dynamic Routing protocols

Routers automatically learn about remote networks

Ex: RIP, EIGRP, OSPF, IS-IS, BGP

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

Host Routing Table

Hosts also have a local routing table.

Usually only contains:

Its own network address (directly connected network)

Default gateway IP address

Hosts usually do not have remote networks in their routing tables

netstat –r

or

route print

This presentation is available at:

http://w3.ualg.pt/~jjose/cisco/

Original presentations from:

http://www.cabrillo.edu/~rgraziani/

Cisco curriculum available at:

http://cisco.netacad.net (Internet Explorer recommended)

After login, under: “Course Materials”