Installing and Administering LAN/9000 Software

Chapter 6 155
Network Addressing
Variable-Length Subnet Addressing
Figure 6-11 Entire Host Field Allocated
To alleviate this restrictive situation, when you use the variable-length
subnetting approach, you can, alternatively, assign subnet numbersfrom
the left of the subnet field and work right. This is implemented using
mirror-image counting. In the mirror image approach, the bits for subnet
numbers are assigned left to right instead of right to left (normal
situation). This would result in the following format:
This will allow for more growth bits between the subnet field and the
host fields as shown below.
0s 1s & 0s
1s & 0s
subnet field host field
subnets can
grow here
Table 6-7 Mirror Image Counting
Traditional
Approach
MirrorImage
Approach
01 10
10 01
011 110
100 001
101 101