Classful Networks - HP ProCurve Secure 7000dl Series Basic Management And Configuration Manual

Host Address
AND
Subnet Mask
=
Network Address
Figure 11-1. Subnet Masks

Classful Networks

In the early days of IP addressing, routing protocols did not always use subnet
masks. The address itself needed to identify which bits were network bits and
which host bits. Classful networks met this need. The first four bits of a classful
IP address identified how many octets belonged to the network address.
Classful network addresses always end evenly at the end of an octet:
Class A networks have 8-bit network addresses. They are identified by a
0 in the first bit. Therefore, the 126 class A networks range from 1.0.0.0 to
126.0.0.0. (127.0.0.0 is reserved for loopback and 0.0.0.0 for default
routes.) Each class A network can accommodate up to 16,777,214 hosts.
Class B network addresses always start with 10 in the first two bits, which
indicates that the network has a 16-bit network address. The 16,384 class
B networks range from 128.0.0.0 to 191.255.0.0. Each network can accom-
modate up to 65,534 hosts.
Class C networks have 24-bit network addresses and always start with 110
in the first three bits. The 2,097,252 class C networks range from 192.0.0.0
to 223.255.255.0. Each class C network can accommodate up to 254 hosts.
Class D networks have 32-bit network addresses and always include 1110
in the first four bits. These networks are used for multicasting and range
from 224.0.0.0 to 239.255.255.255.
You might notice that this schema leaves networks beginning with 1111
undefined. Such networks are called Class E networks and have not been
assigned a specific function.
IP Routing—Configuring Static Routes
172.16.132.99 255.255.0.0
10101100 00010000 10000100 01100011
11111111 11111111 00000000 00000000
10101100 00010000 00000000 00000000
172.16.0.0
Overview
11-5