Although we’ll cover selecting your sharing method later, there is one important thing that you should do when planning to share your Internet connection with your LAN:

Keep your network separated from your ISP’s network!

The key reasons for doing this are security for your LAN and avoiding problems with your ISP.  Read this page for more information.

In the typical case of:

  • an Ethernet-based LAN

  • software-based sharing method

  • external cable or DSL modem connected via 10/100BaseT Ethernet

this will mean having two NICs in the Sharing Server.

I recommend not using two of the same exact NIC (especially 3COM, since users have reported problems with multiple 3COM NICs).   The main reason for this recommendation is that it will be easier for you to tell the NICs apart when you go to configure their Properties in the TCP/IP Control Panel, and when you run the winipcfg utility.

NOTE! Adding the second NIC is probably the most difficult part of getting your Internet connection shared.   Many of the emails that I receive tell long tales of woe and struggle with this step.

Nevertheless, it’s important that you add that NIC!  I’ve put together a page to help you through the process, so give it a look.

When you don’t need a second NIC

The world of Networking has changed since I first wrote this in 1998.  New networking methods have appeared, i.e. wireless, phoneline and powerline, and inexpensive hardware routers have appeared and continue to get even more inexpensive!

These new alternatives create the need for exceptions to the “Two NIC” rule, and require a shift in thinking from “Two NICs” to “Two Network Adapters” in the case of software-based sharing methods.

So here’s the list of “Two NIC” exceptions
(and here’s hoping that you don’t get totally confused by this)…

Exception 1: Internal cable or DSL modems
Since the modem connects directly to your computer’s internal bus (usually via a PCI slot), you don’t need a NIC to connect to it.  Just one NIC (or phoneline or wireless or powerline network card) in the Sharing server is needed to connect to the LAN.

Exception 2: External cable or DSL modems with wireless, phoneline or powerline-based LAN
In this case, one Ethernet NIC is needed to connect to the cable/DSL modem, but since your LAN is not Ethernet based, you don’t need a second NIC.

Exception 3: Hardware router
The router handles the job of keeping your LAN separated from your ISP’s network and also handles sharing the Internet connection.  All you need in this case is one NIC per computer.
(When hardware routers or “residential gateways” or whatever other term the marketing folks come up with, appear that handle Wireless / phoneline / powerline connections directly, this exception should be interpreted to be one network-adapter-of-the-proper-flavor per computer.)

Exception 4: Multiple IPs
The only case in which you can’t create a separate network is if you are using the multiple IP address method of sharing. 
In order for the multiple IP address method to work, all computers that require Internet access need to be connected directly to the ISP’s network, usually via a hub or switch.
In the Multiple IP method, you need only one NIC per computer.

Exception 5: External cable or DSL modems connected via USB
In this case, one Ethernet NIC is needed to connect to the LAN, but since the cable / DSL modem is connected via USB, you don’t need a second NIC.
(Thanks to Jason Aubrey for the tip!)

Keep this information in mind when selecting your sharing method and buying your network hardware.