ShunIPCalc - What is an IP Class ?

What is an IP Class ?

An IP Class is a range of IP addresses. They have been designed to provide an address sheme which accomodates small and large networks.

There are 5 Classes from A to E defined as follows:
Class A - first byte is 1 to 126 decimal (0xxxxxxx in binary)
Class B - first byte is 128 to 191 decimal (10xxxxxx in binary)
Class C - first byte is 192 to 223 decimal (110xxxxx in binary)
Class D - first byte is 224 to 239 decimal (1110xxxx in binary)
Class E - first byte is 240 to 254 decimal (1111xxxx in binary)

IANA allocates only Class A to Class C addresses. Class D is reserved for multicast and Class E for future use.
Adresses begining by 127 (01111111) are reserved for loopback or local testing. The IP 127.0.0.1 is usually implemented as local host loopback IP.

For each IP Class we also define a Network Mask as follows:
Class A - 255.0.0.0 (11111111.00000000.00000000.00000000 in binary)
Class B - 255.255.0.0 (11111111.11111111.00000000.00000000 in binary)
Class C - 255.255.255.0 (11111111.11111111.11111111.00000000 in binary)

The Network part of the IP address is deduced by operating a binary AND between the IP address and the Network Mask of the Class. The binary AND will let "as-is" all binary values combined with "1"s and will zero all others (see "Binary operators").
For example, for Class A:

IP address :

aaaaaaaa.bbbbbbbb.cccccccc.dddddddd

Network mask:

11111111.00000000.00000000.00000000

Binary AND

--------------------------------------

Network :

aaaaaaaa.00000000.00000000.00000000

The Host part of the IP address is deduced by operating a binary AND between the IP address and the Network Mask of the Class inversed (binary NOT).
For example, for the same Class A:

IP address :

aaaaaaaa.bbbbbbbb.cccccccc.dddddddd

Network mask inversed:

00000000.11111111.11111111.11111111

Binary AND

--------------------------------------

Host :

00000000.bbbbbbbb.cccccccc.dddddddd

From this comes:
Class A - the first byte codes the Network ("n"s) and the 3 last bytes are the Host ("h"s) : nnnnnnnn.hhhhhhhh.hhhhhhhh.hhhhhhhh
Class B - the fist two bytes codes the Network ("n"s) and the 2 last bytes are the Host ("h"s) : nnnnnnnn.nnnnnnnn.hhhhhhhh.hhhhhhhh
Class C - the fist three bytes codes the Network ("n"s) and the last bytes are the Host ("h"s) : nnnnnnnn.nnnnnnnn.nnnnnnnn.hhhhhhhh

Here is a summary of number of networks and number of hosts per Class:
Class A - 2^7-2 = 126 networks - 2^24-2 = 16777214 hosts
Class B - 2^14-2 = 16382 networks - 2^16-2 = 65534 hosts
Class C - 2^21-2 = 2097150 networks - 2^8-2 = 254 hosts