Installing and Administering LAN/9000 Software

Chapter 6 157
Network Addressing
Variable-Length Subnet Addressing
Figure 6-13 Example of Variable-Length Subnet Mask
The example below shows how the bits might look in the host address
portion of a Class C address in a network with four subnets using
variable-length subnetting. In this example “g” stands for growth bits
and “h” stands for host bits. The 0’s and 1’s in the host address are
assigned subnet numbers.
If hosts are added to subnet B above, so that all of its remaining “g” bits
become “h” bits, the mask in subnet B also must change. Table 6-9 shows
the results of these changes. Also, two additional subnets have been
added, subnet E and subnet F.
11 111 11 111 11 111 1 1111 111011111 000
subnet portion of host address
growth portion of host address
host portion of host address
Predetermined
boundary
Class C Subnet Mask:
network address
255 . 255 . 255 . 240
Table 6-8 Subnets with Variable-Length Subnet Mask
Subnet Host Address (Bits)
Subnet Mask
(Bits)
A 10gg gghh 1111 1100
B 01gg ghhh 1111 1000
C 110g ghhh 1111 1000
D 001g ghhh 1111 1000