Installing and Administering LAN/9000 Software

154 Chapter6
Network Addressing
Variable-Length Subnet Addressing
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 6-10 Traditional Subnet and Host Field Allocation
In the next example. the entire host field has been allocated. As a result,
only the subnet field can grow.
0s 1s & 0s
0s 1s & 0s
subnet field host field
subnets can
grow here
hosts can
grow here