What do I need to know about IP Addresses

If you have been playing around with computers and networking you will probably have come across IP addresses. So what is an IP address and why do we need them.

Computer Networks work is a similar way to phones. When you want to call your friend on the other side of the city, you need to know their phone number. In most countries the phone number is made up of different area country and area codes that have a logical progression from big and broad down to the most specific.

Computer networks are very similar, the IP address is simply an address for that particular computer. Once you understand how that address is used you will be able to work out how information can move from one computer to another.

First of all there are Private Addresses and Public Addresses. There is really no difference between the two except that the Internet community has declared certain ranges to be Private. The most common two ranges of private addresses are and

So why Public and Private?
Well The public addresses MUST be unique. That means if you have a public address you are the only one in the world that can use it. This is very useful for companies running web servers and email etc. Generally if you want people to be able to connect to your computer, rather than you connection to others, then you will most probably need a Public IP address.

For your home PC, you really only want it to connect to servers to either browse a web-page or get your email from a server that has a public IP address. In this case you can use a private IP for your PC. That same private IP address may be duplicated thousands of times around the world but that is ok so long as all the private networks are kept separate.

So how does information get from My PC to the Internet?
Well there are a few parts to setting up an IP network. For the most basic network you will need an IP address, a netmask and a gateway.
The IP address is the address of your PC, the netmask is a set of numbers that tells the PC how many other IP addresses could be inside your private network, and the gateway is the IP address of the router that connects your network to the next network. Confused yet?

Lets assume that your PC’s IP address is and the netmask is and your gateway is
If you try to communicate with a PC with an IP address of, the PC will work out using the netmask of, that in in your local network and so will send out a message on your network and if that PC is there it will hear it and respond.

If you try to communicate to a PC with an IP address of, then the PC will use the netmask to find out that the in NOT local and therefore will send it to the gateway, in this case Your router on will receive the message and then pass it on to the next network. It may go through hundreds of networks before it finds the right pc.

Routers normally have more than 1 IP address. The reason for this is that their primary job is to connect 2 or more networks together. Your normal Netgear, Linksys or D-Link router that you use at home will have a public IP address (assigned to your by your ISP) and then a private IP address which you may choose (or you may use the default). They are like the door between two rooms, everything that is outside that room must flow through that door.

In most networks your router will have a service called DHCP. “Dynamic Host Configuration Protocol” is a service where the server or router will assign an appropriate IP address to each PC when it is connected to the network and turned on. The PC will send a request message out onto the network and if there is a DHCP server there it will respond with an appropriate IP address details which the PC can then use to communicate.