IP address

in full Internet Protocol address

Number that uniquely identifies each computer on the Internet. A computer's IP address may be permanently assigned or supplied each time that it connects to the Internet by an Internet service provider. In order to accommodate the extraordinary growth in the number of devices connected to the Internet, a 32-bit protocol standard, known as IPv4, began to be replaced by a 128-bit protocol, IPv6, in 2000. See also TCP/IP; domain name; URL.

(Internet Protocol address) The address of a device attached to an IP network (TCP/IP network). Every client, server and network device must have a unique IP address for each network connection (network interface). Every IP packet contains a source IP address and a destination IP address.

Static and Dynamic IP
An IP network is somewhat similar to the telephone network in that you have to have the phone number to reach a destination. The big difference is that IP addresses are often temporary.

Each device in an IP network is either assigned a permanent address (static IP) by the network administrator or is assigned a temporary address (dynamic IP) via DHCP software. Routers, firewalls and proxy servers use static addresses as do most servers and printers that serve multiple users. Client machines may use static or dynamic IP addresses. The IP address assigned to your service by your cable or DSL Internet provider is typically dynamic IP. In routers and operating systems, the default configuration for clients is dynamic IP (see DHCP).

Dotted Decimals
IP addresses are written in "dotted decimal" notation, which is four sets of numbers separated by periods; for example, 204.171.64.2. If you knew the IP address of a Web site, you could enter the dotted decimal number into your browser instead of the domain name (which is why we have DNS!).

Although the next version of the IP protocol offers a virtually unlimited number of unique addresses (see IPv6), the traditional IP address (IPv4) uses a 32-bit number that defines both the network and the host computer. The network class determines how many of the 32 bits are used for the network address, leaving the remaining bits for use as the host number (note the numbers of networks and hosts in the table below). The host number can be further divided between subnetworks and hosts (see subnet mask).

Class A, B and C
Although the computer identifies the class by the first three bits of the address (A=0; B=10; C=110), people identify the class by the first number in the address (see range below). This class-based system has also been greatly expanded, eliminating the huge disparity in the number of hosts that each class can accommodate (see CIDR).

                  Maximum   Maximum    Number of
        Class     Number    Hosts      Bits used in
        Number    of        per        Network/Host
 Class  Range     Networks  Network    ID      ID

 A      1-126          127  16,777,214    7/24
 B      128-191     16,383      65,534   14/16
 C      192-223  2,097,151         254   21/8

        127 reserved for loopback test

Networks, Subnets and Hosts

An IP address is first divided between networks and hosts. The host bits are further divided between subnets and hosts. See subnet mask.

IP - Logical or Physical?
An IP address is somewhat of a hybrid, which can be thought of as either logical or physical, depending on how you view it. It is a unique number assigned to a node, which makes it seem physical, especially because there is so much name-to-IP address resolution going on in the network.

There is also the Ethernet address, which is built into the network adapter. That is indeed physical, and it does not change, which is very typical of physical device names. However, since IP addresses can be dynamically assigned, causing the same client workstation to have a different IP address every day, the IP address seems more like a logical address. Regardless of what it is, it would make a great debate in a computer science class. See logical vs. physical, IPv6, private IP address, TCP/IP abc's and IP on Everything.

(networking)

IP address - (Internet address) The 32-bit number uniquely identifying a node on a network using Internet Protocol, as defined in STD 5, RFC 791. An IP address is normally displayed in dotted decimal notation, e.g. 128.121.4.5.

The address can be split into a network number (or network address) and a host number unique to each host on the network and sometimes also a subnet address.

The way the address is split depends on its "class", A, B or C (but see also CIDR). The class is determined by the high address bits:

Class A - high bit 0, 7-bit network number, 24-bit host number. n1.a.a.a 0 <= n1 <= 127

Class B - high 2 bits 10, 14-bit network number, 16-bit host number. n1.n2.a.a 128 <= n1 <= 191

Class C - high 3 bits 110, 21-bit network number, 8-bit host number. n1.n2.n3.a 192 <= n1 <= 223

DNS translates a node's fully qualified domain name to an Internet address which ARP (or constant mapping) translates to an Ethernet address.