HP Sa3110 - VPN Server Appliance Manual page 15

Hewlett-Packard Company Virtual Private Networking Concepts Guide
For example, "Test Company" is assigned a full class C. This
means "Test Company" can use any address between
205.250.128.1 up to 205.250.128.254. The addresses 205.250.128.0
and 205.250.128.255 are also part of the addresses in the class C
subnet, but are reserved for broadcasting and cannot be assigned
to any devices on the network (often called boundary
addresses).
If you want to break your class C into separate networks, you do
this by varying the last octet of the subnet mask. If you make
your subnet mask 255.255.255.128, your class C is split into 2
parts. This gives you one subnet containing the addresses from
205.250.128.1 to 205.250.128.126 and another subnet containing
205.250.128.129 to 205.250.128.254.
When you work with the full class C, there are 2 boundary
addresses reserved for broadcasts. Every subnet requires 2
addresses for broadcasts. When you split your class C into 2
parts, you must still have broadcast addresses in each subnet.
The first subnet uses 205.250.128.0 and 205.250.128.127 for
broadcasts while the second uses 205.250.128.128 and
205.250.128.255.
When you have the full class C, there are 254 addresses you can
use. Once the class C is split into two subnets, there are 126
addresses in each subnet for a total of 252 addresses.
The following values, if placed in the last octet of the subnet
mask, divide a class C subnet into smaller subnets.
Decimal Value Decimal Value
Decimal Value Decimal Value
(Binary Value) (Binary Value)
(Binary Value) (Binary Value)
255 (1111-1111)
254 (1111-1110)
252 (1111-1100)
248 (1111-1000)
240 (1111-0000)
224 (1110-0000)
Number of Subnets Number of Subnets
Number of Subnets Number of Subnets
254
128
64
32
16
8
TCP/IP Basics Overview
Number of Number of
Number of Number of
Addresses in Each Addresses in Each
Addresses in Each Addresses in Each
Subnet Subnet Subnet Subnet
1
0
2
6
14
30
1-7