HP 200 Series Services And Applications page 114

Internet Protocol Routing Service
IP Addressing Scheme
Suggestion for Assigning Addresses
Once your network number is assigned and you have selected the subnet
mask, you have apportioned the address space that will be available for
additional subnets and for additional hosts in the future. Because it may not
be clear which will increase more—subnets or hosts—you can start with the
scheme described below to reserve the most flexibility for expansion. You
want to reserve the option of slightly changing your subnet mask in the
future (to adjust the relative allocation of space for new subnets and for new
hosts), without having to also change the IP address configured on every
device in your network!
Once you have established your subnet mask, start assigning subnet
numbers at the most significant bit of the subnet field, and host numbers at
the least significant bit of the host field. For example, starting with a class B
address and subnet mask 255.255.255.0, start assigning subnets 128, 64, 192,
32, 160, etc. Within each subnet, start assigning nodes 1, 2, 3, 4, 5, etc. Figure
1 illustrates an example class B network.
1000 0000
0100 0000
1100 0000
0010 0000
1010 0000
0110 0000
1110 0000
132.6.128.2
host
Figure 1. Initial IP Address Asssignments in Router Network 132.6
After it's clear whether more subnets or more host numbers within each
subnet are used, you may be able to change your subnet mask from
255.255.255.0 (using 8 bits for subnets) to 255.255.255.128 (using 9 bits for
subnets) or to 255.255.254.0 (using 7 bits for subnets).
2-62
First Subnets
Binary Decimal
128
64
192
32
160
96
224
132.6.128.1
router
First Hosts
Binary Decimal
0000 0001 1
0000 0010 2
0000 0011 3
0000 0100 4
0000 0101 5
0000 0110 6
0000 0111 7
132.6.64.2
router
132.6.64.1
132.6.192.1