Every computer connected to the Internet has a unique address. This is called its ‘IP address’ and is very similar to the format of your home address. Your home address consists of the town/village and street where you live. Similarly, an IP address has two parts.

The first is a ‘network’ portion containing the address of the local network to which your computer is connected. The second part is the ‘host’ portion containing an identity number for your particular computer on that network.

If you dial-in to your ISP then you may be allocated a different IP address every time you log on. If you have a broadband connection however, you’ll be allocated a ‘static’ IP address that remains the same every time you connect to the Internet.

Information travels backwards and forwards across the Internet in small chunks of data that are called ‘IP packets’. Every IP packet contains all the information it needs to travel from its source computer to its destination computer. This ‘addressing’ information consists of five pieces:

The IP address of the destination computer The IP address of the source computer The destination port number The source port number The transfer protocol used

These IP packets are associated in groups. Each group having the same address information is called a ‘session’. The address information allows millions of sessions to be distinguished from each other.

Now that you understand IP addresses, packets and sessions, you’ll find it a little easier to understand how a firewall works.