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HomeClassless Inter-Domain Routing (CIDR)

IP Address Classes:
There are 5 different address classes which can be determined by its IP address by examining the first 4 bits of the IP address; listed below:

Class A addresses begin with 0xxx, or 1 to 126 decimal.
Class B addresses begin with 10xx, or 128 to 191 decimal.
Class C addresses begin with 110x, or 192 to 223 decimal.
Class D addresses begin with 1110, or 224 to 239 decimal (reserved for multicasting).
Class E addresses begin with 1111, or 240 to 254 decimal (reserved for future use which should not be used for host addresses).

Addresses beginning with 01111111, or 127 decimal, are reserved for loopback and internal testing on a local machine; example "ping".

Private Subnets:
The private addresses,, and are reserved for private networks . They can be used by anyone setting up internal IP networks, since no routers on the Internet will ever forward packets coming from these addresses; as defined in RFC 1918. Also, IP addresses in the range of - are reserved for Automatic Private IP Addressing. - - -


Dotted Decimal

Equivalent Class C

No. of Hosts

2,048 Class C 524288 hosts
1,024 Class C 262144 hosts
512 Class C 131072 hosts
256 Class C (1 Class B) 65536 hosts
128 Class C 32768 hosts
64 Class C 16384 hosts
32 Class C 8192 hosts
16 Class C 4096 hosts
8 Class C 2048 hosts
4 Class C 1024 hosts
2 Class C 512 hosts
1 Class C 256 hosts
1/2 of a Class C 128 hosts
1/4th of a Class C 64 hosts
1/8th of a Class C 32 hosts
1/16th of a Class C 16 hosts
1/32th of a Class C  8 hosts
  4 hosts
  2 hosts
  1 host

/31 mask:

In general the number of available hosts on a subnet is 2n−2, where n is the number of bits used for the host portion of the address.

RFC 3021 <>  specifies an exception to this rule when dealing with 31-bit subnet masks (i.e. 1-bit host identifiers). In such networks, usually point-to-point links <> , only two hosts (the end points) may be connected and a specification of network and broadcast addresses is not necessary.


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