Nangarhar University Faculty of Computer Science

Networking and Communication Dept- Spring 2014

Understanding the TCP/IP Internet Layer

Internet Protocol

• The IP component of TCP/IP determines where packets of data are to be routed based on their destination addresses.

• and IP has certain characteristics related to how it handles this function. This topic describes some of the key functions of IP.

Conti…

• IP uses packets to carry information through the network.

• A packet is a self-contained, independent entity that contains data and sufficient information to be routed from the source to the destination

IP Characteristics

• IP operates at Layer 3 of the OSI model (Network Layer), and Layer 3 of the TCP/IP stack (Internet layer).

• IP is a connectionless protocol in which a one-way datagram is sent to the destination without advance notification to the destination device. The destination device receives the data and does not return any status information to the sending device.

IP Characteristics

• IP uses hierarchical addressing in which the network ID is like a street and the host ID is like a house or office building on that street.

• IP provides service on a best-effort basis and does not guarantee packet delivery. A packet can be misdirected, duplicated, or lost on the way to its destination.

IP Characteristics

• IP does not provide any special features that recover corrupted packets. These services are instead provided by the end systems of the network.

IP Addressing

• To facilitate the routing of packets over a network the TCP/IP protocol suite uses a logical address known as an IP address

Conti…

• Just as physical street addresses are necessary to identify the location

• logical IP addresses are used to identify the location of specific devices on an IP network so that data can reach those network locations efficiently

• The IPv4 address is the most common type of address currently used on the Internet

Conti…

• An IP address is a hierarchical address and consists of these two parts:

• The network address portion (network ID) describes the network of which this IP address is a part. The router maintains information about routes to each network.

• The host address component (host ID) identifies a specific endpoint. These endpoints are the servers, computers, and other devices connected to the network.

Conti…

Conti…

• In any given IP address, some portion of the 32-bit number represents the network and the remaining bits represent the host. While many computers may share the same network address, combining the network address with a host address uniquely identifies any device connected to the network.

• For usability, these 32-bit numbers are broken up into 4 groups of numbers called octets

(1 octet = 8 bits)• Each octet is then represented as a decimal

number between 0 and 255 and separated by a period, or dot.

Conti…

• This scheme is known as “dotted decimal notation.”

• The IP address shown above can be written as 172.16.128.17 and spoken as “172 dot 16 dot 128 dot 17.”

IP Address Classes

• To accommodate different sizes of networks and aid in classifying them, IP addresses are divided into categories called classes. This topic describes the IP address classes and the structure of the IP addresses within them.

IP Address Classes

• Assigning IP addresses to classes is known as class full addressing. The classes were determined during the early days of the Internet by the Internet Assigned Numbers Authority (IANA).

IP Address Classes

• Each IP address is broken down into a network ID and the host ID. In addition, a bit or bit sequence at the start of each address determines the class of the address.

Class A

• The Class A address uses only the first octet (8 bits) of the 32-bit number to indicate the network address.

• The remaining three octets of the 32-bit number are used for host addresses.

• The first bit of a Class A address is always “0.” Since the first bit is a 0, the lowest number that can be represented is 00000000 (decimal 0), and the highest number that can be represented is 01111111 (decimal 127)

Class A

• However, these two network numbers, 0 and 127, are reserved and cannot be used as network addresses. Any address that starts with a value between 1 and 126 in the first octet of the 32-bit number is a Class A address.

Class B

• The Class B address uses two of the four octets (16 bits) to indicate the network address

• The remaining two octets specify host addresses.

• The first 2 bits of the first octet of a Class B address are always binary 10

Class B

• Starting the first octet with binary 10 ensures that the Class B space is separated from the upper levels of the Class A space. The remaining 6 bits in the first octet may be populated with either 1s or 0s.

Class B

• Therefore, the lowest number that can be represented with a Class B address is 10000000 (decimal 128), and the highest number that can be represented is 10111111 (decimal 191). Any address that starts with a value in the range of 128 to 191 in the first octet is a Class B address.

Class C

• In a Class C address, the first three octets (24 bits) of the IP address identify the network portion, with the remaining octet reserved for the host portion

• A Class C address begins with binary 110 Therefore, the lowest number that can be represented is 11000000 (decimal 192), and the highest number that can be represented is 11011111 (decimal 223)

• If an address contains a number in the range of 192 to 223 in the first octet, it is a Class C address.

Reference

• http://www.tutorialspoint.com/ipv4/ipv4_address_classes.htm