IP address

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Historical classful network architecture
Class First octet in binary Range of first octet Network ID Host ID Number of networks Number of addresses
A 0XXXXXXX 0 - 127 a b.c.d 27 = 128 224 = 16,777,216
B 10XXXXXX 128 - 191 a.b c.d 214 = 16,384 216 = 65,536
C 110XXXXX 192 - 223 a.b.c d 221 = 2,097,152 28 = 256
D 1110XXXX 224 - 254 a.b.c.d e 223 = 2,100,199 29 = 512

Classful networks have been replaced by Classless Inter-Domain Routing (CIDR) since 1993. CIDR also provides a network address and host address. CIDR does not have classes, which means network and host address sizes don't have to be in octets.

An IPv4 Address in CIDR notation looks like

The slash and number represent the amount of bits that the network id uses, in this case 24 or 4 octets.

IP Version 6[change | change source]

Because IPv4 is only 32 bits, the number of available addresses will run out. To prevent this, an organization called the Institute of Electrical and Electronics Engineers (IEEE) created IP Version 6 (IPv6), which will eventually replace IPv4.

IP Version 6 uses 16 octets, or 128 bits in total. Octets in IPv6 are written in hexadecimal, and separated by colons (:). An IPv6 address might look like this:


An IPv6 address can be long and this can lead to mistakes when typing them into the computer or writing them down. There are two ways in which an IPv6 address can be made shorter without leaving anything out:

  • Leading zeroes can be left out: 2001:0db8:00b8:0008:0000:0000:0000:0001 becomes 2001:db8:b8:8:0:0:0:1
  • Any number of sequential, all-zero 'chunks' may be compressed to simply ::. This can be done only once in the same address: 2001:0db8:0000:0000:0000:0000:0000:0001 could be written as 2001:db8::1

Other versions[change | change source]

Versions before IPv4 were experimental and never widely used. Version 5 was used exclusively for the Internet Stream Protocol, which was also never widely used.