What is IP Address Classes?

Ans : Originally, the IP address space was divided into a few fixed-length structures called
address classes. The three main address classes are class A, class B, and class C. By
examining the first few bits of an address, IP software can quickly determine the
class, and therefore the structure, of an address. IP follows these rules to determine
the address class:

• Class A: If the first bit of an IP address is 0, it is the address of a class A
network. The first bit of a class A address identifies the address class. The next 7
bits identify the network, and the last 24 bits identify the host. There are fewer
than 128 class A network numbers, but each class A network can be composed of
millions of hosts.

• Class B: If the first 2 bits of the address are 1 0, it is a class B network address.
The first 2 bits identify class; the next 14 bits identify the network, and the last 16
bits identify the host. There are thousands of class B network numbers and each
class B network can contain thousands of hosts.

• Class C: If the first 3 bits of the address are 1 1 0, it is Internet Protocol a class C network
address. In a class C address, the first 3 bits are class identifiers; the next 21 bits
are the network address, and the last 8 bits identify the host. There are millions of
class C network numbers, but each class C network is composed of fewer than
254 hosts.

• Class D: If the first 4 bits of the address are 1 1 1 0, it is a multicast address.
These addresses are sometimes called class D addresses, but they don’t really
refer to specific networks. Multicast addresses are used to address groups of
computers all at one time. Multicast addresses identify a group of computers that
share a common application, such as a video conference, as opposed to a group of
computers that share a common network.

• Class E: If the first four bits of the address are 1 1 1 1, it is a special reserved
address. These addresses are called class E addresses, but they don’t really refer
to specific networks. No numbers are currently assigned in this range.

IP addresses are usually written as four decimal numbers separated by dots (periods).
Each of the four numbers is in the range 0-255 (the decimal values possible for a
single byte). Because the bits that identify class are contiguous with the network bits
of the address, we can lump them together and look at the address as composed of full
bytes of network address and full bytes of host address. If the value of the first byte
is:

• Less than 128, the address is class A; the first byte is the network number, and the
next three bytes are the host address.

• From 128 to 191, the address is class B; the first two bytes identify the network,
and the last two bytes identify the host.

• From 192 to 223, the address is class C; the first three bytes are the network
address, and the last byte is the host number.

• From 224 to 239, the address is multicast. There is no network part. The entire
address identifies a specific multicast group.

• Greater than 239, the address is reserved.

The IP address, which provides universal addressing across all of the networks of the
Internet, is one of the great strengths of the TCP/IP protocol suite. However, the
original class structure of the IP address has weaknesses. The TCP/IP designers did
not envision the enormous scale of today’s network. When TCP/IP was being

designed, networking was limited to large organization’s that could afford substantial
computer systems. The idea of a powerful UNIX system on every desktop did not
exist. At that time, a 32-bit address seemed so large that it was divided into classes to
reduce the processing load on routers, even though dividing the address into classes
sharply reduced the number of host addresses actually available for use. For example,
assigning a large network a single class B address, instead of six class C addresses,
reduced the load on the router because the router needed to keep only one route for
that entire organization. However, an organization that was given the class B address
probably did not have 64,000 computers, so most of the host addresses available to
the organization were never assigned.
The class-structured address design was critically strained by the rapid growth of the
Internet. At one point it appeared that all class B addresses might be rapidly
exhausted. To prevent this, a new way of looking at IP addresses without a class
structure was developed.