Fact: Comcast doesn't support any OS except Windows & Mac. It doesn't mean that Unix/Linux users can not access internet with Comcast services; but it means Comcast refuses to trouble shoot any issues on *nix platform ie., *nix users are on their own to fix any of the issues

In general, Comcast technician leaves a CD-ROM to activate the Comcast account, once the installation is done. The activation wizard of the CD takes care of registering the modem, activating the account for us. Unfortunately it works only on Windows and Mac platforms

However Comcast is kind enough to provide web interface for activating the account (some people refer it as "registering the modem"; Comcast refers it as "Activating the ISP"). Comcast uses DHCP (Dynamic Host Configuration Protocol) for IP assignment. So, if we have a DHCP client running on the machine, it contacts the Comcast's DHCP server(s) and obtains a valid IP, DNS info etc., ie., As soon as the cable modem installation is done, our machine will be on the network (to verify, simply try arp <any well known hostname>); but we can't actually access any of the public web sites due to the pending registration. Since we are in the network of Comcast, we will be able to access few web sites of Comcast, with the help of Comcast's proxy server. URLs for Comcast's primary proxy servers are as follows:

1. https://sas.r1.attbi.com serves Oregon & Washington regions
   
2. https://sas.r2.attbi.com serves California except Sacramento
   
3. https://sas.r3.attbi.com serves Texas
   
4. https://sas.r4.attbi.com serves Utah, Colorado, Wyoming, Montana & Sacramento (CA)
   
5. https://sas.r5.attbi.com serves Indiana, Illinois & Michigan
   
6. https://sas.r6.attbi.com serves Ohio, Connecticut & Pennsylvania

Activation steps are as follows:

1. Configure your browser to use the Comcast's proxy server for your geographic region (see above for the list)
   
   1. HTTP & SSL proxies must be configured to use the proxy server of your region
      
   2. 8000 is the port for both HTTP & SSL proxies
      
   3. Fill out "No Proxy for:" field with the name of the proxy server for your region without the string "https://sas." from the actual URL
   
   
2. Now try to access the proxy server URL for your region. Hopefully you should be able to see a page with two fields: Account Number and Registration Code. Account Number is the Subscriber Number and Registration Code is the number (ID) that you may find under "Service Name and Address" field of the invoice you received from Comcast.
   
3. As you proceed to the next step, you will be also be prompted to setup your Comcast e-mail account
   
4. Once the device (modem) is registered, the web page shows a link to restart the cable modem. Go ahead and restart the modem. If you are using a router, you need to restart the modem as well. To restart the router: turn the power off, wait for a min and then turn it back on.
   
5. Now your account is activated; but you still can not access public web sites due to Comcast's proxy server configuration. Since we do not need it anymore, go back to web browser's proxy configuration window and choose "Direct connection to the internet"
   
6. If you can not access internet even at this point, try to restart the network or the machine depending on your expertise level. Even if it didn't help, try calling Comcast support center and find out the the proxy server, that you need to use in your region

PS:
The above steps are equally applicable for other platforms as well; but Unix, Linux were explicitly added to the title for Google feed

Thanks to:
James Dean Palmer