|The Local Area Network (LAN)|
The Connection Between Your Modem/Router And Your ComputerThe Plun 'n Play approach with modern Windows computers - particularly if it's running XP - works so well that, when you first connect your modem/router to the computer via a LAN (ethernet) cable, it just works! Following the modem/router manufacturer's instructions, you plug it in, power it up and connect to it using your web bowser with something like: http://192.168.1.1/, type in a default Username & Password and there's the router's "Home" page.
It's then simple enough to enter the details specific to your ISP: Your ISP username and login (another one to remember!) and some other details such as PPPoA which don't need to mean anything to you. When you re-start the modem/router, its lights flash and you're connected to the internet before you know it.
It really is virtually plug 'n play - except, of course, for your ISP-specific details which the modem/router couldn't be expected to know until you tell it! So how does it fit together?
DHCPWindows XP installed the software necessary to run the Local Area Network when it first detected 'ethernet' hardware on your system. This may have been when a plug-in PCI card was fitted or, most common these days,was already built onto the computer's motherboard and was detected when the operating system was installed.
If you select Control Panel then double-click Network Connections, you should see a window similar to the one shown on the far left, in the picture below.
When you first connect the computer to the modem/router, the computer detects that it is part of a Local Area Network and immediately asks other devices on the network for a LAN IP address. The modem/router is configured by the manufacturer to respond. It already knows its own LAN IP address (again, pre-configured by its manufacturer) so responds to the computer with the next address in the sequence.
Other than for this initial request, it is only by using this address provided by the modem/router that the computer and the modem/router will be able to communicate with each other. The modem/router is able to supply the computer with the requested LAN IP address because the modem/router has a special mechanism, known as a DHCP server, built into it.
DHCP is an acronym for "Dynamic Host Configuration Protocol" which simply means that the modem/router is able to assign the computer a LAN IP address when one is requested. If a second computer is plugged in to another LAN port on the modem/router, it can automatically assign that computer a different LAN IP address - if that computer requests one.
How Are The Addresses Assigned?Every device connected to your Local Area Network has to have a unique LAN IP address. The modem/router is assigned a default address by the manufacturer. He can't choose any address he pleases, though. There are certain address 'blocks' reserved specifically for this purpose so he must use one of those. By far the most common address is 192.168.0.1 as used by most Netgear modem/routers. Other manufacturers use an address in the 10.0.0.xx range and others use 192.168.8.1
Although this address isn't "cast in stone", in practice there isn't usually much point in changing it. When the modem/router uses its DHCP server to assign a LAN IP address to a computer, it generally begins with the next available address in its range. So, if the modem/router is 192.168.0.1, it would assign the first computer as 192.168.0.2, the second as 192.168.0.3 and so on. Note that the individual LAN IP Addresses assigned to a group of computers will depend on the order in which they are connected to the modem/router. We'll return to his last point later because it is important if there is more than one computer and one of them is running a server.