Using Comcast High Speed Internet with a Broadband Router

This page explains how to get Comcast High-Speed Internet to work with a broadband router.  (A router might also be referred to as a "residential gateway" or "firewall".  Basically, it's a box that plugs into your cable or DSL modem and allows you to connect other computers to the internet, either by ethernet or 802.11 wireless, a.k.a. WiFi.)

Brief summary: you have to configure the router to clone the MAC address of the computer you used to run the Comcast installation software, and use DHCP to assign the router an IP address.  After that, everything seems to work as expected.

I believe this information to be correct at the time of writing.  However, I make absolutely no guarantees about whether this information will work for anyone else: use it at your own risk.  Also, I live in Prince George's county, Maryland.  This information may not be correct for Comcast customers in other parts of the country.

I use a D-Link DI-614+ router.  However, this information should apply to any kind of router.

Last revised $Date: 2004/10/13 14:34:38 $.

Comments to daveho@cs.umd.edu.

What to do

In these steps I've highlighted the most important points in boldface.

1. Install the cable modem so it is plugged in, connected to the cable, etc.
2. Connect a PC running Windows to the cable modem using ethernet. 
3. Find the MAC address of the ethernet adapter.  To find the MAC address, run the command ipconfig /all from a DOS window.  Information about each network adapter in the system will be listed.  Look for the line labeled "Physical Address" for the ethernet adapter you are using to connect to the cable modem.  That is the MAC address.  Write it down for later reference.
4. Run the Comcast software to register your account.  When this process completes you should be connected to the internet.  Test that you are able to view web sites, etc.
5. Shut down Windows.  Unplugging the cable modem and plugging it back in is probably not a bad idea.
6. Now, turn on your router, and connect your PC to it (by either ethernet or wireless).  However, don't connect the router to the cable modem yet.  Log into the administrative interface of the router.  Usually, this is done using a web browser.  See your router's documentation for more information.
7. Configure the router so the MAC address of its WAN interface is the same as the MAC address you wrote down in step 3.  Often, this function is called "Clone MAC Address".  Essentially, the Comcast installation software programs the cable modem so it will only work when connected to a single computer: the one you used to set up your broadband connection.  Cloning the MAC address tricks the modem into thinking the router is that computer.
8. Configure the router to use DHCP.  Generally, you will find this option in the part of the administrative interface that offers other settings such as "PPPOE" and "Static IP".  You should pick the "DHCP" option.
9. Now, you can connect the router's WAN port to the cable modem.  At this point, you should be able to connect to the internet from other computers through the router.

Notes

Using a router may violate the Comcast terms of service.  However, all consumer broadband routers have a built-in firewall function that protects your computer(s) from worms and other forms of attack.  I strongly feel that for this reason alone, using a router is an absolute necessity.  Being able to share the connection wirelessly between multiple computers is a nice additional benefit, of course.