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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:

Simple Internet Connection Sharing

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.

"With Internet Connection Sharing enabled, a computer takes on many of the roles of a traditional low-end router."

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:

Internet Connection Sharing with Multiple Computers

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.

Article C2687 - June 13, 2006 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

Sam A.
June 15, 2006 5:23 PM

I have ICS set up because I can only get dial-up in my area. One thing I noticed is that every once in a while the client computers will no longer be able to access the internet. I have been unable to find out why it is doing this but have figured out how to make it work again. Just go and disable ICS and then enable again. It works for while then I have to do that again.

Obviously it gets annoying doing this every week. Does anybody know of a permanent fix?

June 17, 2006 12:34 PM

We find that our main computer that is connected to the modem (a cable modem, as it happens) must be booted up first. After waiting a minute or two we turn on the second computer and wait a few minutes for it to communicate with the first computer. After doing this, ICS works fine. If we restart one computer then we must also restart the other computer, otherwise we lose the internet on the second computer.

April 23, 2007 9:47 AM

i would like to know how to get to ICF and ICS step by step.

secondly i would like to know the required hardware and software to connect to the internet.

james morgan
December 22, 2007 11:03 AM

On my Host puter I lose all of the places where one would set up ics. I was working ok. It says in the cadvance tab that wmi is possible corrupted
How does one restore using restore does not correct.

September 2, 2008 7:56 AM

Dear Leo,

I have encountered the problem to set-up my network system.

I have one laptop have a WLAN device and now is using the 3G wireless modem connect to the internet. My partner need to connect to internet as well using another laptop.

Please advice me that the way to solve it.

Thanks. Leo

Appreciated and regards,

January 11, 2009 12:24 PM

Hi Leo,

I am not being able to turn off ICS on my Vista laptop. I don't understand why it is turned on in the first place, it is greyed out. Can you please tell me how I can turn off ICS?

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