Introduction to TCP/IP and routing : Introduction to IP : Architecture of IP : Addressing
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1.2.1. Addressing

An IP address is represented by a 32 bit integer, theoretically giving 4,294,967,296 different addresses (though there are less available).

The address is almost always displayed as four bytes separated by a dot (.):

  172.16.1.2

Associated to this address are a number of bits, called the netmask, which determine the network number.

Originally, the IP address space was divided into classes, of type A, B, and C.

From RFC 1020 (http://www.pasteur.fr/other/computer/RFC/10xx/1020):

 The first type of address, or class A, has a 7-bit network number
 and a  24-bit local address. The  highest-order bit is set  to 0.
 This allows 128 class A networks.

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |0|   NETWORK   |                Local Address                  |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Class A Address

 The second type of address, class  B, has a 14-bit network number
 and a 16-bit local address. The two highest-order bits are set to
 1-0. This allows 16,384 class B networks.

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 0|           NETWORK         |          Local Address        |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Class B Address

 The third type  of address, class C, has a  21-bit network number
 and a 8-bit  local address. The three highest-order  bits are set
 to 1-1-0. This allows 2,097,152 class C networks.

                     1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |1 1 0|                    NETWORK              | Local Address |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                              Class C Address

Note: the notion of classes (class A, class B, and class C) are obsolete -- they have been replaced with CIDR -- Classless Inter Domain Routing.


Introduction to TCP/IP and routing : Introduction to IP : Architecture of IP : Addressing
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