An IPv4 address is a 32-bit address that is usually represented
in dotted decimal notation, with a decimal value representing each
of the 4 octets (bytes) that make up the address. For example:
The IP address consists of a network address and a host
address. Within the internet, the network addresses are assigned
by a central authority, the Network Information Center (NIC).
The portion of the IPv4 address that is used for each of these addresses
is determined by the class of address. There are four commonly
used classes of IPv4 address (see Figure 1).
Figure 1. Classes of
IP Addresses
The class of address of the IPv4 network is determined from the
first 4 bits in the first octet of the IP address. Figure 2 shows how the class of address is determined.
Figure 2. Determining
the Class of an IP Address
32-bit address xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
Class A 0xxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
min 00000000
max 01111111
range 0 - 127 (decimal notation)
Class B 10xxxxxx xxxxxxxx xxxxxxxx xxxxxxxx
min 10000000
max 10111111
range 128 - 191 (decimal notation)
Class C 110xxxxx xxxxxxxx xxxxxxxx xxxxxxxx
min 11000000
max 11011111
range 192 - 223 (decimal notation)
Class D 1110xxxx xxxxxxxx xxxxxxxx xxxxxxxx
min 11100000
max 11101111
range 224 - 239.255.255.255
As shown in Figure 2, the value of the
bits in the first octet determine the class of address, and the class
of address determines the range of values for the network and host
segment of the IP address. For example, the IP address 9.67.97.2 would
be a class A address, since the first 2 bits in the first octet contain B'00'.
The network part of the IP address is 9 and the host part of
the IP address is 67.97.2.
Refer to RFC 1166 - Internet Numbers for more information
about IP addresses. Refer to RFC 1060 - Assigned Numbers for
more information about reserved network and host IP addresses, such
as a network broadcast address.
