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IP Addresses

Simple Ethernet Network

IP addresses or "all those numbers" arent anywhere near as frightening as they may first appear. In this section we will examine the essentials of IP addresses and how to use them. An IP address is a number used to identify a computer when it is connected to a network. We could consider it to be very much like a street address for a house or building, after all if you dont have a street address the postman would find it hard to deliver your mail, wouldnt he? Packets of data find their way in a similar fashion. An IP address is a group of four numbers seperated by dots, for example or Using our street address comparison we could think of the format of the address as . Figure 1 is an example of a simple local area network (or LAN). There are three computers connected to each other via network cables and a switch. Note that the IP addresses for these machines are different by one on the very last number (the "housenumber"). The first is, the second and so on if necessary to the limit of 254 addresses. These computers are on the same network and as such are able to communicate with each other. If one of the computers were changed to it would no longer be able to see the other computers and communicate with them.

On the internet IP addresses are assigned to servers so that things like email and web sites work properly. For these servers to work properly they must have unique IP addresses, ones that are not used anywhere else in the entire world. Because of this there are special ranges of IP's that are designated for use in places like private LAN's (Local Area Networks) such as we intend to build. They are:

Windows Vista Network Configuration - Click to Enlarge

When configuring the IP address on most devices you will likely be asked for a subnet mask or netmask. Subnetting is a way of deciding how many IP addresses will be on your network. The subnet mask used almost exclusively on this website will be which means in our above example that we can use every address between and and they will be part of the same network and able to see each other. As subnetting is unnecessary in the average home or small office network we shall not enter into any further explanation of it and its many uses.

Simple Ethernet Network with Gateway

Another option that most computers or devices ask for is a gateway or default gateway in their configuration. Some devices may also refer to it as the router address. What the device really wants to know is which address on the network should it send requests for data that is outside of your local area network. Figure 2 has the computers IP address as If it needs to connect to any computer on your LAN it will simply transmit and receive the data, knowing that any computer that has an address of - will be connected locally and if its switched on and working it will be listening. If it has to get data from outside the LAN (from any other address) it will send the request to the default gateway instead, which in this example is The default gateway will be a device such as an ADSL or cable modem that will provide a connection to other networks such as the internet.

The final IP address configuration option you will commonly encounter is the DNS server or domain name server address. DNS is the process by which human friendly names such as are converted into IP addresses so that the computer can request the data it requires in a method it understands. When you type into your web browser (say internet explorer) your computer will query a DNS server, usually your internet service providers, and ask it what the IP address of is. The DNS server will reply with the appropriate IP address and your computer will then be able to ask its gateway for the required information, in this case this website. Nameservers are usually specified by their IP address and if your computer doesnt have at least one specified you wont go far on the internet. If you dont know your internet service providers DNS server addresses contact them and ask them, they should be quite happy to tell you what they are.

So we have learnt the four things that are required to configure IP address settings for most computers and devices. IP address, subnet mask, gateway and nameserver address are all most things need to actually communicate over your network and beyond. What if you had lots of computers and had to configure them all? There is an easier way! If we look at figure 2 we can see an option to obtain an IP address automatically and also obtain nameserver addresses automatically. This is achieved by dhcp (Dynamic Host Configuration Protocol). Instead of configuring each PC manually we can use a dhcp server to provide each computer its address instead. DHCP servers are built into most routers and access points. The tutorials section includes some examples of configuring a DHCP server but essentially all the server needs to know is the range of addresses it should hand out (eg - 100), the subnet mask, the gateway address and the name server address. Sound Familiar? To avoid problems you should make sure that only one dhcp server runs on each network.



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