One of the fundamental features of classful IP addressing is that each

address contains a self encoding key that identifies the dividing point

between the network prefix and the host number. For example, if the

first two bits of an IP address are 1 0, the dividing point falls between

the 15th and 16th bits. This simplified the routing system during the

early years of the Internet because the original routing protocols did not

supply a deciphering key or mask with each route to identify the length

of the network prefix. 

Class A Networks (/8 Prefixes)

Each Class A network address has an 8 bit network prefix, with the

highest order bit set to 0 (zero) and a 7 bit network number, followed

by a 24 bit host number. Today, Class A networks are referred to as

 /8s  (pronounced  slash eight  or just  eights ) since they have an 8 

bit network prefix. 

A maximum of 126 (27  2) /8 networks can be defined. The calculation

subtracts two because the /8 network 0.0.0.0 is reserved for use as the

default route and the /8 network 127.0.0.0 (also written 127/8 or

127.0.0.0/8) is reserved for the  loopback  function. Each /8 supports a

maximum of 224  2

(16,777,214) hosts per network. The host calculation subtracts two

because the all 0s (all zeros or  this network ) and all 1s (all ones or

 broadcast ) host numbers may not be assigned to individual hosts. 

Since the /8 address block contains 231 (2,147,483,648 ) individual

addresses and the IPv4 address space contains a maximum of 232

(4,294,967,296) addresses, the /8 address space is 50 percent of the total

IPv4 unicast address space. 

Class B Networks (/16 Prefixes)

Each Class B network address has a 16 bit network prefix, with the two

highest order bits set to 1 0 and a 14 bit network number, followed by a

16 bit host number. Class B networks are now referred to as  /16s  since

they have a 16 bit network prefix. 

A maximum of 16,384 (214 ) /16 networks can be defined with up to

65,534 (216 2) hosts per network. Since the entire /16 address block

contains 230 (1,073,741,824) addresses, it represents 25 percent of the

total IPv4 unicast address space. 

Class C Networks (/24 Prefixes)

Each Class C network address has a 24 bit network prefix, with the

three highest order bits set to 1 1 0 and a 21 bit network number, fol 

lowed by an 8 bit host number. Class C networks are now referred to as

 /24s  since they have a 24 bit network prefix. 

A maximum of 2,097,152 (221 ) /24 networks can be defined with up to

254 (28 2) hosts per network. Since the entire /24 address block con 

tains 229 (536,870,912) addresses, it represents 12.5 percent (or one 

eighth) of the total IPv4 unicast address space. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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