IP address

Internet Protocol address. A unique identifier for a computer on a TCP/IP network. IP addresses consist of four numbers separated by periods. Each number may range from 0 to 255. Thus, is an example of a valid IP address.

Every computer that connects to the Internet must be represented by a public IP address. Normally, each computer has its own public IP; but computers that run behind NAT boxes (such as cable/DSL routers) actually share the use of a single public IP. In such a setup, the NAT box is the middleman; it talks to computers on the Internet using a public IP, and relays information to computers on the LAN using private IPs.

The term “private IP” refers to any address within these ranges:

  • -
  • -
  • -

By design, private IPs are not valid on the Internet. If they were—if, in fact, all IPs were valid for public use—then network administrators would be forced to pick IPs at random for use on their private networks. If these networks were ever connected to the Internet, there would be numerous conflicts, which would defy the concept of “unique identifiers” and thus prevent the Internet from functioning any better than a toaster without coils. Hence, it is vitally important to make a distinction between public and private IP addresses.

Of course, a single private IP (, for example) may be simultaneously used on thousands of private networks around the world; but since these networks are never directly connected to each other, a conflict never occurs.

IP masquerading

An old form of NAT built into the Linux operating system.


Internet Protocol Security. A set of protocols intended to allow secure transmissions over public TCP/IP networks; part of the specification for VPN. To be used through a router, IPSec must be supported by the router’s firmware.


Interrupt Request Line. A hardware channel over which devices can send signals.

An IRQ conflict occurs when two devices try to use the same channel. IRQ conflicts are less common with the advent of Plug 'n Play, but they still happen occassionally when a user tries to add a second NIC to a computer.


Internet Service Provider. A company that provides Internet access to its customers. In most cases, this is accomplished thru the use of dial-up modems; but many ISPs now offer faster service with DSL and/or cable modems.