3Com OfficeConnect 140M User Manual page 91

Class A. The highest order bit is set to 0, giving a
seven-bit network number and a 24-bit device
address. You can have 125 networks with 16,777,214
devices per network, and the addresses are in the
range 001.xxx.xxx.xxx to 126.xxx.xxx.xxx (where xxx
represents the device portion).
Class B. The two highest order bits are set to 10,
giving a 14-bit network number and a 16-bit device
address. You can have 16382 networks with 65,534
devices per network, and the addresses are in the
range 128.001.xxx.xxx to 191.254.xxx.xxx (where xxx
represents the device portion).
Class C. The three highest order bits are set to 110,
giving a 21-bit network number and a 8-bit device
address. You can have 2,097,152 networks with 254
devices per network, and the addresses are in the
range 192.000.001.xxx to 223.255.254.xxx (where xxx
represents the device portion).
Class D. This is used as a multicast address. The four
highest order bits are set to 1110. Multicasting is
used to send an IP datagram to all members of a host
group. A host group is composed of a set of devices
identified by a single IP address. The addresses are in
the range 224.000.000.000 to 239.255.255.255.
Subnets
You can further divide your IP network into sub
networks. Support for sub networks is important
because the number of bits assigned to the device
portion of an IP address limits the number of devices
that may be addressed on any given network. For
example, a Class C address is restricted to 254
devices.
If you have a small network (less than 254 devices)
then you may decide not to have sub networks.
A subnet mask is used to divide the device portion of
the IP address into two parts:
The first part identifies subnet number.
The second part identifies the device on that
subnet.
IP Addresses B-3