Modem DHCP with static IP's - Help!!

Question

Hi,

I need help and business phone support has lost their minds, so I hope an admin on the forum can help!

We have a normal 10/50 business service with 5 static IP's. When we plug a server into the modem with one of the static IP's pre-assigned, then that works fine. When we plug a device into one of the other modem ports with DHCP enabled, it gets an address from the 10.1.10.x range from the builtin DHCP server. This has worked fine for years.

Recently our leased cable modem fried so we had a service call to replace it. The new model we got is a little different - I see more options in the IPv6 area for example; not sure how to tell what model we have.

However after the cable modem was replaced, the built-in DHCP server stopped functioning, so the only devices on our network that have Internet access now are the 5 devices with static IP's.

We tested this and reported it to the tech while he was there, and he said "modem configuration is the customer responsibility" and pointed us to the config page where the internal DHCP server is set. That page showed that DHCP is enabled and the address range is assigned properly (these are all defaults, we didn't change anything). But it didn't work. The tech didn't know what to do so he mumbled something about his next appointment and left.

Then we called business support and got even worse service. The rep said "you can't have DHCP if you have static IP's that was never supposed to work, and if it did your old modem was misconfigured."

A couple of years ago, one of our modems stopped providing DHCP service, and at that time the phone support people knew exactly what we were talking about and arranged a modem swap; the tech who showed up knew all about it too. So to hear phone support now say "that's never been supported" is frustratiing beyond belief.

So what can we do? Is it really true that Comcast has dropped support for DHCP if we have static IP's? Do we need to request an older model modem? Is there a level 2 support line where somebody actually knows what DHCP is?

We were happy that replacing our modem restored our internet access, but not so happy that it was only restored for 5 systems and everybody else is dead in the water.

Help!

epoxymg · Accepted Answer

Let me reply to my own post.

I called phone support back. Apparently if you don't have static IP's, then the modem does DHCP on the WAN side (makes sense), and that's what phone support thought I was talking about. I had to have the agent sign into my modem, go to the LAN page, and look at the check box. Then the agent said "oh, THAT DHCP" and then we were on the same page (the first agent never figured out what I was talking about).

Got the modem replaced a second time and it still didn't work, but this installer was much more persistent. He called *his* tech support and after a long time on the phone they decided that my modem needed to "have the profile rebuilt". They did the profile rebuild and finally my LAN DHCP started working again!

So bottom line, this is NOT a feature that Comcast has dropped support for as my first phone agent told me. But this function does stop working on the business cable modems from time to time, and when it does, you have to be very persistent to get it fixed; Comcast support isn't really ready to understand what you are saying and is very likely to think you are talking about WAN DHCP, not LAN DHCP. Mentioning that sometimes "rebuilding the profile" fixes this issue would probably help a lot.

Eparlin · Answer

This doesn't sound like Comcast's fault. It sounds like the problem just needs simplified. Your DHCP agent on the gateway stopped working. The 5 devices with the static IPs have nothing to do with that and caused the first technician the issue because he was absolutely correct - it's either DHCP on a segment or static, not both. That's an internet standard since Moses created the internet. The second tech that you think made so much sense saying 'DHCP on WAN' is absolutely clueless and just called a service tech out cause it was the easoest thing to do. It was the right call I believe but too bad the tech will not be corrected. I suspect your 5 statically assigned hosts are on a separate segment than your DHCP hosts. If not, you got bigger issues than the DHCP server/agent problem. Glad it's fixed, just wanted to clarify that for you for future reference.

Eparlin · Answer

But...if you don't mean static but mean reserved IPs (as in reserved from the DHCP pool) then can be on same segment obviously. Was taking your static to be a 'technically correct' static. This leads to confusion by phone techs as well.

epoxymg

Modem DHCP with static IP's - Help!!

epoxymg

Eparlin

Eparlin