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Internet Connection Sharing connects multiple computers to the internet through a single computer. Internet Connection Sharing is much like a router.
I have a computer connected directly to the internet. I've since purchased a second computer, and would like to connect it as well. Is there a way I can do that without buying yet more equipment?
My standard answer to this question is to just bite the bullet and purchase a broadband router. In fact my earlier article How should I set up my home network? outlines exactly that setup.
But that's not always practical. For example if all you have is a dial-up connection to the internet, most routers aren't going to work for you.
That's where something called "Internet Connection Sharing" comes in.
Internet Connection Sharing, or ICS, is exactly what its name implies - a way for a computer to share its connection to the internet.
The first problem we encounter when setting up ICS is that the computer whose internet connection we want to share must allow connections to both the internet, and to the other computers that want to share the connection. In the case of dial-up, that typically means the computer needs both a modem or phone connection to connect to the internet, as well as a network connection to connect to the other computers on a local area network or LAN. In the case of a broadband connection to the internet that might mean the computer would actually need two network adapters - one for the internet and one for the LAN.
So depending on how you connect to the internet, and what network connections are available on your computer, you might need to purchase and install a network adapter.
The very simplest use of ICS looks somewhat like this:
Computer A, in the middle, is somehow connected to the internet - possibly through a dial-up connection.
Computer B's network adapter is connected to the network adapter of Computer A using a special ethernet cable refereed to as a "cross-over" or "reversing" cable.
Computer A is configured with ICS enabled. Among other things, that means that Computer A will assign an IP address to Computer B. All internet requests made by computer B are routed through computer A and then send on to the internet, and the data returning follows the same path in reverse.
The down side? There are a couple. The most obvious is that Computer A must be on and running in order for Computer B to be able to access the internet. If Computer A crashes, shuts down, or is simply overloaded, it can affect its ability act as the "middle man" for the other computers attempting to connect to the internet. In addition, while Computer B is "hidden" behind Computer A, Computer A is sitting directly on the internet, so it better have the appropriate firewalls and other internet safety software installed and running properly.
And of course if you're sharing something as slow as a dial-up connection, it only gets slower - perhaps much slower - if multiple computers are attempting to use it at the same time.
Let's look for a second what Computer A is doing in this configuration:
Computer A assigns IP addresses to the locally connected computers such as Computer B
Computer A performs Network Address Translation (NAT) for all data traveling between those local computers and the internet.
By virtue of NAT, Computer A is acting as a firewall to incoming traffic, protecting the local computers.
Computer A is performing some rudimentary routing - determining if incoming traffic is destined for itself, or for other computers on the LAN.
Sounds familiar? It should. With Internet Connection Sharing enabled, a computer takes on many of the roles of a traditional low-end router.
So what if we want to connect more than one computer? Add a hub to the mix:
The hub simply acts as a way to connect multiple computers to the shared port on the ICS-enabled Computer A.
And of course, if you're going to buy a hub anyway, then if at all possible I'd recommend purchasing a router instead, configuring your LAN "normally", and avoiding the limitations of ICS all together.
If ICS looks to be the appropriate solution for you, rather than duplicate the configuration here I'll point you at this article: Setting up Internet Connection Sharing which has step by step instructions.
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