An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there." The designers of the Internet Protocol defined an IP address as a 32-bit number and this system, known as IPv4, is still in use today. However, due to the enormous growth of the Internet and the predicted depletion of available addresses, a new version IPv6, using 128 bits for the address, was developed in 1995.

IPv6 was standardized as RFC 2460 in 1998, and its deployment has been ongoing since the mid-2000s. IP addresses are binary numbers, but they are usually stored in text files and displayed in human-readable notations, such as 172.16.254.1 (for IPv4), and 2001:db8:0:1234:0:567:8:1 (for IPv6). Classful network design allowed for a larger number of individual network assignments and fine-grained subnetwork design. The first three bits of the most significant octet of an IP address were defined as the class of the address.

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Three classes (A, B, and C) were defined for universal unicast addressing. Depending on the class derived, the network identification was based on octet boundary segments of the entire address. Each class used successively additional octets in the network identifier, thus reducing the possible number of hosts in the higher order classes. The Internet Assigned Numbers Authority (IANA) manages the IP address space allocations globally and delegates’ five regional Internet registries to allocate IP address blocks to local Internet service providers and other entities.

Several address ranges are reserved for "Special Use". These addresses all have restrictions of some sort placed on their use, and in general should not appear in normal use on the public Internet. The overview below briefly explains the purpose of these addresses – in general they are used in specialized technical contexts. They are described in more detail in RFC 5735. ■"Private Use" IP addresses:

10.0.0.0 - 10.255.255.255

172.16.0.0 - 172.31.255.255

192.168.0.0 - 192.168.255.255

These address blocks are reserved for use on private networks, and should never appear in the public Internet. There are millions of private networks (for example home firewalls often use them). People can use these address blocks without informing us, so we have no record of who uses which of these addresses.

The point of private address space is to allow many organizations in different places to use the same addresses, and as long as these disconnected or self-contained islands of IP-speaking computers (private networks) are not connected, there is no problem. If you see an apparent attack, or spam, coming from one of these address ranges, then either it is coming from your local environment, your ISP, or the address has been "spoofed". The Private addresses are documented in RFC 1918.