IP addresses are divided in to 'classes', based on the decimal value represented in the first octet. This class
definition is referred to as the First Octet Rule. There are five classes of IP addresses: classes A, class B,
class C, class D; and class E, but only class A, B and C addresses are used to identify devices connected to
the Internet. Class D addresses are used for multicasting, and Class E addresses are reserved for
experimental use. The subnet mask is related to the IP address class. Thus, once the IP address class is
known, the default routing mask is also known.
The IP address classes and their related subnet masks are:
• Class A addresses range from 0.0.0.0 through 126.255.255.255 and use a default subnet mask of
255.0.0.0. In Class A addresses, the first octet is used as for the network ID while the last three octets are
used for the host ID. In other words, the first 8 bits of the subnet mask are all 1s, hence a subnet mask of
255.0.0.0. As a result, networks that use Class A addresses can theoretically support a maximum of 256
networks and 16,581,375 (255x255x255) hosts, however, the first and the last address cannot be used.
The first address is the network address and the last address is the broadcast address. For example, a
network with an IP address of 10.10.11.12 has a network ID of 10.0.0.0, the fist address, and a broadcast
address of 10.255.255.255, the last address. Thus networks with a Class A IP address space can support a
maximum of 254 networks (28-2) and 16,777,214 hosts (224-2). Consequently, Class A addresses are
used for a few networks with a very large number of hosts on each network.
• Class B addresses range from 128.0.0.0 through 168.255.255.255 and 170.0.0.0 through
191.255.255.255. These addresses use a default subnet mask of 255.255.0.0. In Class B addresses, the
first two bits are used as for the network ID while the last two bits are used for the host ID. As a result,
networks that use Class B addresses can support a maximum of 65,534 networks (216-2) and 65,534
hosts. Consequently, Class B addresses are used for a reasonable number of medium sized networks.
Note: IP addresses with a first octet of 127, i.e. 127.0.0.0 through
127.255.255.255 do not fall in either the Class A address range or the Class
B address range. IP addresses that have a first octet of 127 are reserved for
diagnostics purposes.
• Class C addresses range from 192.0.0.1 through 223.225.225.225 and default subnet mask of
255.255.255.0. In Class C addresses, the first three bits are used as for the network ID while only the last
bit is used for the host ID. As a result, networks that use Class C addresses can support a maximum of
16,777,214 networks and 254 hosts. Consequently, Class C addresses are used for a large number of
networks with a relatively small number of hosts on each network.
• Class D addresses are in the range 224.0.0.0 through 239.255.255.255. These addresses are reserved for
multicast transmissions.
• Class E addresses are in the range 240.0.0.0 through 254.255.255.255. These addresses are reserved for
experimental use.
Note: InterNIC has reserved a number of IP address range, including 169.0.0.1
through 169.253.255.254, which has been reserved by for future use;
0.0.0.0, which was originally defined for use as a broadcast address; and
127.0.0.0, which is used as the loopback address. 128.0.0.0, 191.255.0.0,
192.0.0.0, and 223.255.255.0 also are reserved.