TCP/IP Configuration and Management Manual

Configuring the NonStop TCP/IP Subsystem
TCP/IP Configuration and Management Manual—427132-004
3-12
Configuration 2: Startup Files for a Host in a Subnet
Addressing Environment
This plan will be simple to implement if you assign the subnet addresses beginning
with the high-order bits and host IDs beginning with the low-order bits. In such a case,
the first few subnet addresses would be 128, 64, 192, and so on. In this case, you
might need to change the subnet number and the subnet mask.
In this sample environment, suppose that you need to combine subnets 128.30.192.0
and 128.30.64.0 due to an administrative consolidation. If you assigned the subnet
addresses and host addresses as recommended, you may have enough room for the
added hosts by simply adjusting your subnet mask.
For example, the current subnet mask looks like this in binary form:
11111111 11111111 11111111 00000000
For a Class B address, the first two octets determine the network address. In the
subnet mask above, you use the third octet to specify the subnet address. These
subnets are 192, 64, and 32. The corresponding host addresses are numbered
consecutively throughout the network when possible. You are numbering the subnets
using the high order bits first and numbering the hosts using the low order bits first. So,
the binary representations of two host addresses, for example, HOST1 and HOST3,
respectively are:
10000000 00011110 11000000 00000010
10000000 00011110 01000000 00000110
Their addresses in dotted decimal form are:
128.30.192.2
128.30.64.6
In the third octet, you use the high-order bits first (128, 192, and 64); in the fourth octet
for the host ID, you use the low-order bits first (1, 2, 3, and so on). By using this subnet
and host addressing method, when you need to increase the number of subnets or
hosts beyond the octet boundary, you have room to do so.
However, if you started numbering the subnets with the low order bits first (1, 2, 3, and
so on), then needed to place more that 254 hosts on that subnet, you would have to
reconfigure each host on the subnet.
If you leave room for growth in the number of subnets and hosts, you substantially
reduce the number of changes and reconfiguration needed on each host.