How to Share a Cable Modem Connection

There are two basic ways to share a cable modem connection:

1. If you are allowed to connect more than one PC directly to the cable modem, you can 'direct connect'.
2. If you do not have permission to connect more than one PC, or if you try to connect more than you have permission to connect, you have to use a 'Proxy or NAT translation'.

You will also have to use the 2nd method for internal cable modems and for some systems that need a regular modem for the return signal (one way cable modem).

Important: Do not connect more computers to the same hub with the cable modem then you are allowed to. Some cable modems will transmit all traffic received on the hub to the cable network. This can disturb the cable network and lead to high packet loss for other users.

The following sketch shows how to connect a larger LAN to a cable modem. Use two hubs, only connect the allowed number of PCs (in this case 3) to the cable modem (external) hub. One of the three machines can work as a gateway for the rest of the network. This machine (red screen with smiley in the sketch) has to have two Ethernet cards installed, one of which is connected to the external, and the other to the internal hub.

(The 'smiley' computers will be able to connect to the cable modem directly using their own IP addresses. They will need to have the login software installed if your cable modem operator requires a special login software (e.g. RoadRunner). You could use some proxy software on the gateway computer (red screen smiley, the one that has the two Ethernet cards installed) to allow the local net PCs to connect to the Internet.)

Further Reading:

- Manual of your Ethernet card (it includes some information on maximum cable length/type)

- your cable modem documentation.

- Tim Higgins FAQ about sharing cable modems

- Linux IP Masquerading Mini-Howto (in case you would like to use Linux on the gateway machine).