LIS 525 - IP Numbers


IP (Internet Protocol) was first standardized in 1981. The specification required each system to be assigned a unique, 32-bit address for each internetwork to which it was attached. The first part of an IP address, the network prefix, identified the network on which the host resided, and the second part, the host number identified the particular host.

The relative size of the two parts depended on the class of network. Having different network classes provided for different network sizes. The main classes were Class A, Class B, and Class C. Class A used 8 bits for the network prefix; Class B, 16 bits; and Class C, 24 bits; and, in each case, the rest of the IP number identified the specific host. The class was identifiable by specific bits in the network prefix: the first bit in Class A was always 0; the first two bits in Class B were always 1 and 0; the first three bits in Class C were always 1, 1, 0. The two other classes were Class D, used for multicasting, with the first four bits 1, 1, 1, 0, and class E, used for experimental purposes, with the first four bits 1, 1, 1, 1. This original scheme is no longer strictly valid, and some ranges of IP numbers have been reassigned to different networks.

IP addresses are usually displayed in dotted decimal notation: the address is divided into 4 8-bit segments and each is represented as a decimal number between 0 and 255, with dots separating the numbers. You can easily identify the original class of network as follows from the first decimal number in this notation:
From To Class Example
0 127 A (network prefix, 64; host number: 85.73.36)
128 191 B (network prefix, 143.126; host number: 211.222)
192 223 C (network prefix, 200.52.72; host number: 150)
224 239 D
240 255 E
Thus, you can see, for example, that the UWO network (which had its IP number range assigned according to the original scheme) is a Class B network, because its IP numbers start with 129.

Larger individual networks may choose to identify local subnetworks with the initial bits of the host number to improve efficiency of routing.

Some numbers are reserved for special purposes; for example, for the default route ("this network"), for the "loopback" function, for "broadcast".

IP Number Registration

Until 2002, The American Registry for Internet Numbers (ARIN) handled assignment of IP numbers throughout the Americas and sub-Saharan Africa, and the RIPE Network Coordination Centre (RIPE NCC) covered Europe, the Middle East, and other parts of Africa. In 2002, responsibility for South and Central America and parts of the Carribean was transferred to the Latin American and Caribbean IP address Regional Registry (LACNIC). In 2005, responsibility for all of Africa, including offshore islands such as Mauritius, was reassigned to AfriNIC. APNIC (Asia Pacific Network Information Centre) covers the Asia Pacific from Afghanistan to Pitcairn Island.

ARIN provides a list of countries and territories, indicating which registration service has responsibility for each. (For example, it tells us that Antarctica is covered by ARIN.)

ARIN provides a search form that allows you to find out who has been assigned a given block of IP numbers, though the information returned may not be entirely up-to-date. For example, if you search on any of the IP numbers in the UWO block you get a response like the following:
OrgName:    University of Western Ontario 
OrgID:      UWO
Address:    Information Technology Services
Address:    Natural Sciences Building
Address:    Rm 108
City:       London
StateProv:  ON
PostalCode: N6A-5B7
Country:    CA

NetRange: - 
NetName:    UWO-NET
NetHandle:  NET-129-100-0-0-1
Parent:     NET-129-0-0-0-0
NetType:    Direct Assignment
NameServer: NS1.UWO.CA
NameServer: NS2.UWO.CA
NameServer: NS3.UWO.CA
RegDate:    1987-10-27
Updated:    2006-10-18

RAbuseHandle: NOC310-ARIN
RAbuseName:   Network Operations Centre 
RAbusePhone:  +1-519-661-3525

RNOCName:   Network Operations Centre 
RNOCPhone:  +1-519-661-3525

RTechHandle: BB6694-ARIN
RTechName:   Borowski, Brian 
RTechPhone:  +1-519-661-2111

OrgTechHandle: NOC310-ARIN
OrgTechName:   Network Operations Centre 
OrgTechPhone:  +1-519-661-3525
Similar search forms are provided by other IP number authorities, including RIPE, APNIC, LACNIC, and AfriNIC.


DHCP (Dynamic Host Configuration Protocol) is a protocol for assigning dynamic IP addresses to devices. Many ISPs use dynamic IP addressing for their users, whose machines therefore do not have fixed IP addresses.

How to Find the IP Number of a Windows Workstation

Windows 98

Select "Run" from the "Start" menu, type winipcfg in the edit box and press Enter. You should see an "IP Configuration" dialog. Click on "OK" to close it.

Windows XP

Select "Settings|Control Panel" from the "Start" menu; in Control Panel, double click on "Network Connections"; in the "Network Connections" dialog, click on the connection (e.g., "Local Area Connection"). If the "Common Tasks" pane is visible on the left, scroll down to the bottom of this pane and look for the IP number under "Details"; Otherwise, right click on the connection; select "Status"; in the status dialog, the IP address should be displayed at the "Support" tab. When done, close all unneeded dialogs and windows.

Special Uses for IP Numbers

Because ranges of IP numbers are assigned geographically, servers can be configured to provide different content automatically to users from different parts of the world, as in the case of Google, or even to provide no substantive content to undesirable requesters, as was the case for a while with Showtime.

If logging is enabled, IP numbers can be used to compile rough statistics on the geographical locations of a site's visitors.

For More Information


Last updated October 4, 2007.
This page maintained by Prof. Tim Craven
E-mail (text/plain only):
Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
Canada, N6A 5B7