Every computer connected to the Internet is identified by a unique address known as the IP address (Internet Protocol address). An IP address is a 32 bit binary number, which for ease is usually expressed as four decimal numbers, each representing eight bits, separated by periods e.g. 206.43.44.130.

You are probably thinking, and quite correctly, that surely there are more computers than there are IP addresses. Correct! To overcome this limitation you will find that most ISPs have a limited number of IP addresses that they dynamically assign to their users each time an individual user connects.

In short, each time you connect through your ISP you will likely have a different IP address allocated to you each time.

Next time you are online, check it out. If you are using Windows, open a DOS window and type in the command "ipconfig." You will be given your current IP address. Each time you connect you will likely be given a different address. You will also notice that the IP allocated to you will be contained within the range of addresses that has been allocated to the ISP itself.

If you are using Mac you can see the same information in the TCP/IP settings in your Control Panel.

Please note, you may have been allocated a unique IP address therefore no one else can use the same IP address as you. It is yours to use at will.

New IP Systems
However, the process of dynamically allocating IP addresses does not ensure ongoing availability of sufficient addresses to satisfy demand. To overcome this limitation, there are a number of methods being invoked. These include:

CIDR - Classless Inter Domain Routing
A scheme that replaces the older system based on classes A, B, and C. With CIDR, a single IP address can be used to designate many unique IP addresses. A CIDR IP address looks like a normal IP address except that it ends with a slash followed by a number, called the IP prefix. For example:

172.200.0.0/16

The IP prefix specifies how many addresses are covered by the CIDR address, with lower numbers covering more addresses. An IP prefix of /12, for example, can be used to address 4,096 former Class C addresses. The figure below shows how a typical /16 (Class B) Internet address can be expressed in dotted decimal notation.



IP version 6 (also known as IPng - IP Next Generation)
An IP v6 address is basically an extension of the current IP address format. Whereas the current format is a representation of a 32-bit number, the new IP version 6 is a 128-bit number which expands the possible range of addresses significantly.