3Com corebuilder 3500 Implementation Manual page 265

Network Portion
The location of the boundary between the network part and the host
part depends on the class that the central agency assigns to your
network. The three primary classes of IP addresses are A, B, and C:
Class A address — Uses 8 bits for the network part and 24 bits for
the host part. Although only a few Class A networks can be created,
each can contain a very large number of hosts.
Class B address — Uses 16 bits for the network part and 16 bits for
the host part.
Class C address — Uses 24 bits for the network part and 8 bits for
the host part. Each Class C network can contain only 254 hosts, but
many such networks can be created.
The high-order bits of the network part of the address designate the IP
network class. See Table 38.
Table 38 How Address Class Corresponds to the Address Number
Address Class
A
B
C
Subnet Portion
The IP address can also contain a subnet part at the beginning of the host
part of the IP address. Thus, you can divide a single Class A, B, or C
network internally, allowing the network to appear as a single network to
other external networks. The subnet part of the IP address is visible only
to hosts and gateways on the subnet.
When an IP address contains a subnet part, a subnet mask identifies the
bits that constitute the subnet address and the bits that constitute the
host address. A subnet mask is a 32-bit number in the IP address format.
The 1 bits in the subnet mask indicate the network and subnet part of the
address. The 0 bits in the subnet mask indicate the host part of the IP
address, as shown in Figure 44.
High-order Bits
0nnnnnnn
10nnnnnn
11nnnnnn
Key Concepts 265
Address Number
(Decimal)
0-127
128-191
192-254