HP-UX LAN Administrator's Guide (Feburary 2007)

Network AddressingDRAFT COPY
Variable-Length Subnet Addressing
Chapter 7118
Variable-Length Subnet Addressing
For the most efficient use of address space and maximum flexibility in increasing/decreasing
the size of your subnets, Hewlett-Packard recommends variable-length subnet addressing. To
maximize the possibilities offered with this new approach, you should utilize mirror image
counting, as described in this section, to select subnet numbers.
In the past, a network administrator typically assigned values to the subnet number and host
address fields in numerical order. For example, within a given subnet, hosts were numbers 1,
2, 3, etc. and within a given network, the subnets were numbered 1, 2, 3, etc. The result was
that some bits on the right side of the subnet field and host field were “ones and zeros” and
some bits on the left side of the subnet and host fields were “all zeros” for all subnets and
hosts. As shown below, the “all zeros” bits represented room for growth, and the “ones and
zeros” bits represented bits already consumed by growth.
Figure 7-9 Traditional Subnet and Host Field Allocation
0s
1s & 0s
0s 1s & 0s
subnet field host field
subnets can
grow here
hosts can
grow here