Mumbai University > Information Technology > Sem 4 > CN

Marks: 6M

Year: May 2016

IP address

• Communication at the network layer is host-to-host (computer-to-computer); a computer somewhere in the world needs to communicate with another computer somewhere else in the world. Usually, computers communicate through the Internet. The packet transmitted by the sending computer may pass through several LANs or WANs before reaching the destination computer.

• For this level of communication, we need a global addressing scheme; we called this logical addressing. The term IP address means logical address in the network layer of the TCP/IP protocol suite. An IPv4 address is a 32-bit address that uniquely and universally defines the connection of a device (for example, a computer or a router) to the Internet. IPv4 uses 32-bit addresses, which means that the address space is 232232.

• In classful addressing, the address space is divided into five classes: A, B, C, D, and E. Each class occupies some part of the address space.

  • Netid and Hostid

• In classful addressing, an IP address in class A, B, or C is divided into netid and hostid. These parts are of varying lengths, depending on the class of the address.

Mask

• Although the length of the netid and hostid (in bits) is predetermined in classful addressing, we can also use a mask (also called the default mask), a 32-bit number made of contiguous 1s followed by contiguous as. The masks for classes A, B, and C are shown in Table below. The concept does not apply to classes D and E.

• The mask can help us to find the netid and the hostid. For example, the mask for a class A address has eight 1s, which means the first 8 bits of any address in class A define the netid; the next 24 bits define the hostid.

• The last column of above Table shows the mask in the form 1n where n can be 8, 16, or 24 in classful addressing. This notation is also called slash notation or Classless Interdomain Routing (CIDR) notation. The notation is used in classless addressing,

Subnetting

• During the era of classful addressing, subnetting was introduced. If an organization was granted a large block in class A or B, it could divide the addresses into several contiguous groups and assign each group to smaller networks (called subnets) or, in rare cases, share part of the addresses with neighbors. Subnetting increases the number of 1s in the mask.