Installing and Administering LAN/9000 Software

142 Chapter6
Network Addressing
Subnet Addresses
release Hewlett-Packard systems now support variable-length subnet
addressing. The advantages of using variable-length subnet addressing
over fixed-length subnet addressing include the following:
• Allows the local administrator to easily increase/decrease the size of
individual subnets when variable-length subnet addressing is used in
conjunction with a non-contiguous numbering system (and/or ranges
of numbers for each subnet that are non-contiguous).
The mirror-image counting feature, which will be discussed in the
following section, also allows for more possibilities in the numbers
that can be assigned to individual subnets.
• Reduces the amount of rework that network planners will have to do
on network design after the initial plan has been completed.
Fixed-length subnets were easy to implement,but growth restrictions
often meant that it was necessary to invest more time whenever a
change was made to a subnet after it had been originally designed.
This new feature also makes it possible to have more than one subnet
mask on a network.
As described previously, an internet address can be represented as four
fields separated by a period, each of which represents 8 bits of the overall
address.
The subnet address is based on the host address portion of the internet
address. The host address portion subdivides into subnet number and
host number fields to accommodate a given number of subnetworks and
a given number of nodes per subnetwork. The size of the subnet number
field is determined by the subnet mask, which is explained later in this
section. The subnet number field must contain a minimum of one bit.
In the example below, the IP address, 192.6.12.33, has a subnet number
of 1.
Refer to Figure 6-6 earlier in this chapter for an illustration of how the
subnet mask is ANDed with the IP address to form the subnet number.