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.

IP address

(Internet Protocol address) The address of a device attached to an IP network (TCP/IP network). Every client, server and network device is assigned an IP address, and every IP packet traversing an IP network contains a source IP address and a destination IP address.

Every IP address that is exposed to the public Internet is unique. In contrast, IP addresses within a local network use the same private addresses; thus, a user's computer in company A can have the same address as a user in company B and thousands of other companies. However, private IP addresses are not reachable from the outside world (see private IP address).

Logical Vs. Physical
An IP address is a logical address that is assigned by software residing in a server or router (see DHCP). In order to locate a device in the network, the logical IP address is converted to a physical address by a function within the TCP/IP protocol software (see ARP). The physical address is actually built into the hardware (see MAC address).

Static and Dynamic IP
Network infrastructure devices such as servers, routers and firewalls are typically assigned permanent "static" IP addresses. The client machines can also be assigned static IPs by a network administrator, but most often are automatically assigned temporary "dynamic" IP addresses via software that uses the "dynamic host configuration protocol" (see DHCP). Cable and DSL modems typically use dynamic IP with a new IP address assigned to the modem each time it is rebooted.

The Dotted Decimal Address: x.x.x.x
IP addresses are written in "dotted decimal" notation, which is four sets of numbers separated by decimal points; for example, 204.171.64.2. Instead of the domain name of a Web site, the actual IP address can be entered into the browser. However, the Domain Name System (DNS) exists so users can enter computerlanguage.com instead of an IP address, and the domain (the URL) computerlanguage.com is converted to the numeric IP address (see DNS).

Although the next version of the IP protocol offers essentially an unlimited number of unique IP addresses (see IPv6), the traditional IP addressing system (IPv4) uses a smaller 32-bit number that is split between the network and host (client, server, etc.). The host part can be further divided into subnetworks (see subnet mask).

Class A, B and C
Based on the split of the 32 bits, an IP address is either Class A, B or C, the most common of which is Class C. More than two million Class C addresses are assigned, quite often in large blocks to network access providers for use by their customers. The fewest are Class A networks, which are reserved for government agencies and huge companies.

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


 NETWORKS VERSUS HOSTS IN IPV4 IP ADDRESSES

                  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.

(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.