IP address

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IP address

[¦ī′pē ə‚dres]
(computer science)
A computer's numeric address, such as 128.201.86.290, by which it can be located within a network.

IP address

(networking)
(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.

IP address

(Internet Protocol address) The address of a connected device in an IP network (TCP/IP network). Every desktop and laptop computer, server, scanner, printer, modem, router, smartphone, tablet and smart TV is assigned an IP address, and every IP packet traversing an IP network contains a source IP address and a destination IP address.

Public and Private Addresses
For homes and small businesses, the entire local network (LAN) is exposed to the Internet via one public IP address. Large companies use several public IPs.

In contrast, the devices within the network use private addresses not reachable from the outside world, and the routers and firewalls make sure of it. The same private IP address ranges are used in every network. Therefore, a computer in company A can be assigned the same private IP address as a computer in thousands of other companies. See private IP address and NAT.

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 built into the hardware (see MAC address). An IP address can be changed because it is a logical address that is assigned, but the physical MAC address is hardwired into the device and cannot be changed.

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 "dynamic" IP addresses via software (see DHCP). Cable and DSL modems are typically assigned dynamic IPs for home users and static IPs for business users.

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 website, 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 VS. HOSTS IN IPV4 IP ADDRESSES    Range   Maximum    Maximum  BitsClass       Networks   Hosts    Net/Host

 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.
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