Mumbai University > Electronics and Telecommunication > Sem 6 > Computer Communication and Telecom Network

Marks: 10M

Year: May 2015

Netid and Hostid:

i. In classful addressing, an IP address in classes A, B, and C is divided into netid and hostid.

ii. These parts are of varying lengths, depending on the class of the address. Figure1 shows the netid and hostid bytes. Classes D and E are not divided into netid and hostid.

iii. In class A, 1 byte defines the netid and 3 bytes define the hostid. In class B, 2 bytes define the netid and 2 bytes define the hostid. In class C, 3 bytes define the netid and 1 byte defines the hostid.

Class A:

i. Since only 1 byte in class A defines the netid and the leftmost bit should be 0, the next 7 bits can be changed to find the number of blocks in this class.

ii. Therefore, class A is divided into 27 = 128 blocks that can be assigned to 128 organizations (the number is less because some blocks were reserved as special blocks).

iii. However, each block in this class contains 16,777,216 addresses, which means the organization should be a really large one to use all these addresses. Many addresses are wasted in this class. Figure 2 shows the block in class A.

Class B:

i. Since 2 bytes in class B define the class and the two leftmost bit should be 10 (fixed), the next 14 bits can be changed to find the number of blocks in this class.

ii. Therefore, class B is divided into 214 = 16,384 blocks that can be assigned to 16,384 organizations (the number is less because some blocks were reserved as special blocks).

iii. However, each block in this class contains 65,536 addresses. Not so many organizations can use so many addresses. Many addresses are wasted in this class. Figure 3 shows the blocks in class B.

Class C:

i. Since 3 bytes in class C define the class and the three leftmost bits should be 110 (fixed), the next 21 bits can be changed to find the number of blocks in this class.

ii. Therefore, class C is divided into 221 = 2,097,152 blocks, in which each block contains 256 addresses that can be assigned to 2,097,152 organizations (the number is less because some blocks were reserved as special blocks). Each block contains 256 addresses.

iii. However, not so many organizations were so small as to be satisfied with a class C block. Figure 4 shows the blocks in class C.

Class D:

i. There is just one block of class D addresses. Each address in this class is used to define one group of hosts on the Internet.

ii. When a group is assigned an address in this class, every host that is a member of this group will have a multicast address in addition to its normal (unicast) address. Figure 5 shows the block.

Class E:

There is just one block of class E addresses. It was designed for use as reserved addresses, as shown in Figure 6.