I have my own email server in my house, on a static IP address. I have had business DSL for over a decade, but I also now have Comcast business Internet. I want to transition from the DSL to the Comcast, and I have some questions.

I have a domain name, my own mail server, and a firewall (a PC with two network interfaces, running Devil-Linux). I need to make sure I understand how to set up the Comcast cable box, and how to set up my firewall.

First, do I need to change any settings in the cable box? Currently I have only used the cable box by plugging in a laptop, with the laptop doing DHCP. I think I can leave the box alone but I would like to make sure.

Second, I'm not sure I understand the instructions Comcast gave me for setting up the firewall. My DSL provider gave me the following information: static IP address, net mask, gateway, and two DNS servers. Comcast gave me: static IP address, routable static IP address, net mask, and two DNS servers, and told me to put the "static IP address" as the "gateway" on the firewall. Is this just Comcast-speak here? Does "routable static IP address" mean the same thing as "static IP address" in my DSL setup, the end-point address that I should publish in the DNS MX records for my email server? Or should I publish the "static IP address", and Comcast will then route all its traffic over the cable box?

My plan is: first, I'm going to configure another firewall, so I have one firewall for the DSL and one for the Comcast (rather than madly editing settings to switch back and forth). Then I will publish the new Comcast static IP address as a backup email server address in the DNS MX records, wait a while to let it propagate, and then switch my home over from the DSL to the Comcast. Then I'll change DNS to make that the primary mail address and the DSL the secondary, let that go a while and make sure it seems reliable. Then I'll remove the DSL from the DNS MX records completely, and finally shut down the DSL service.

(I thought about keeping the DSL as a backup, but the reason I'm leaving DSL is that it has become unreliable; and I have heard that Comcast business Internet is reliable.)

Final question, any advice for me? Anything you think might be useful, helpful, or educational. Thanks.


I switched from DSL to Comcast static IP a couple years ago. I'm pretty happy with the service. Comcast gave me two IP addresses, xx.xx.xx.185 as my IP address and xx.xx.xx.186 as my gateway, which goes to web interface on the modem. They never used the term "routable static IP address" when I talked to them.

It's a little hard to guess which static IP they've given you is the gateway and which is your address but the easy way to tell would be to try opening them as http://xx.xx.xx.xx from a browser to see what you get. Offhand, especially given what they told you, I'm betting the "routable static IP" is your IP and the other static IP (presumably non-routable, meaning it's not accessible outside Comcast's network) is your gateway. But if that doesn't work, try the reverse.

I definitely understand the belt-and-suspenders approach to making the switch and wanting to make sure your new DNS entries have propagated before ditching your DSL. But it's not like it used to be. I can remember it taking a week in the late 90s for DNS entries to propagate. Today, it's usually minutes. Good luck, but I think you'll do fine. If you do need it, my experience is that Comcast's tech support people are pretty good.

  • Oh, hello again! And thanks for the answer. Yes, it did occur to me to just try connecting to both addresses and see which one works. The Comcast support person said the "routable IP address" would be a piece of equipment connected to the cable box. I was wondering if anyone else knows this odd jargon... I tried Google searching for "routable static IP address" with unsatisfying results. Maybe this isn't Comcast-speak but just weird jargon made up by the person to whom I was speaking on the phone? – steveha Sep 17 '12 at 1:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.