System information

The Network Layer
Book Title
7-94
Figure 7-3 Class A, B, and C Address Formats
IP networks can also be divided into smaller units, called subnets. Subnets provide extra flexibility
for network administrators. For example, assume that a network has been assigned a Class B address,
and all the nodes on the network currently conform to a Class B address format. Then assume that
the dotted decimal representation of this network’s address is 128.10.0.0 (all zeros in the host field
of an address specifies the entire network). Rather than change all the addresses to some other basic
network number, the administrator can subdivide the network using subnetting. This is done by
borrowing bits from the host portion of the address and using them as a subnet field, as shown in
Figure 7-4.
Figure 7-4 Subnet Addresses
If a network administrator has chosen to use 8 bits of subnetting, the third octet of a Class B IP
address provides the subnet number. For example, address 128.10.1.0 refers to network 128.10,
subnet 1; address 128.10.2.0 refers to network 128.10, subnet 2; and so on.
The number of bits borrowed for the subnet address is variable. To specify how many bits are used,
IP provides the subnet mask. Subnet masks use the same format and representation technique as IP
addresses. Subnet masks have ones in all bits except those bits that specify the host field. For
example, the subnet mask that specifies 8 bits of subnetting for Class A address 34.0.0.0 is
255.255.0.0. The subnet mask that specifies 16 bits of subnetting for Class A address 34.0.0.0 is
255.255.255.0. Both of these subnet masks are shown in Figure 7-5.
0Class A
Network
Host
1Class B
Network
0
Host
1Class C
Network
1 0
Host
1Class B
Network
Class B address, before subnetting
Class B address, after subnetting
0
Host
1Class B
Network
0
Subnet Host