Classful IP Addressing

When IP was first standardized in September 1981, the specification

required that each system attached to an IP based Internet be assigned

a unique, 32 bit Internet address value. Systems that have interfaces to

more than one network require a unique IP address for each network

interface. The first part of an Internet address identifies the network on

which the host resides, while the second part identifies the particular

host on the given network. This creates the two level addressing hierar 

chy that is illustrated in Figure 3. 

F I G U R E   3 .   Tw o   L e v e l   I n t e r n e t   A d d re s s   S t r u c t u re

In recent years, the network number field has been referred to as the

network prefix because the leading portion of each IP address identifies

the network number. All hosts on a given network share the same net 

work prefix but must have a unique host number. Similarly, any two

hosts on different networks must have different network prefixes but

may have the same host number. 

Primary Address Classes

To provide the flexibility required to support networks of varying sizes,

the Internet designers decided that the IP address space should be

divided into three address classes Class A, Class B, and Class C. This is

often referred to as classful addressing. Each class fixes the boundary

between the network prefix and the host number at a different point

within the 32 bit address. The formats of the fundamental address

classes are illustrated in Figure 4. 

F I G U R E   4 .   P r i n c i p l e   C l a s s f u l   I P A d d re s s   F o r m a t s

3