Have I screwed up subnetting?

Factual Questions
1

I have two computers at home that are tied into a D-Link DI-624. It connects, in turn, to a cable modem. Both PCs connect easily to the D-Link router and get IP addresses of the form

IP: 192.168.0.xxx
Subnet mask: 255.255.255.0

but when I tell the D-Link to go get an IP address from the cable modem, it says it has timed out and won’t go do this simple favor for me. This keeps me from accessing the internet wirelessly (a source of great frustration for my pregnant and basically housebound wife). But I know that there is an IP waiting at the cable modem – that’s how I have connected to SDMB to ask this question! I unplug the cable running to the router and plug it into my PC instead, and use

ipconfig /renew

to get a new address for the ethernet card. Voila, my PC now has two addresses. On the wireless card it has 192.168.0.xxx, but on the old ethernet card it has 24.126.57.xxx, and I connect flawlessly to the internets! I noticed, for no good reason, that both addresses are in the same subnet mask. Is it possible that I am somehow feeding bad info to the DI-624? I’ve even tried telling the DI-624 what address is waiting for it at the cable modem (by forcing static IP) with no luck.

In short, why won’t my DI-624 get an IP address from the cable modem when the PC in the same room can get the IP address every time?

2

MAC cloning issue?

Some cable companies bind their DHCP duties to one MAC address. Your D-Link provides for this annoyance in the way of MAC cloning. Just make sure your MAC clone is from the proper computer when you do it.

3

If I’m understanding your question right, check if it’s got a DNS server manually specified in the problem system.

That’s normal. All my systems I’ve ever used have 255.255.255.0 as their subnet mask.

4

I’ve tried getting the D-Link to pretend it’s every piece of hardware in my bin: I’ve given it MAC addresses from LAN cards, wireless cards, even the MAC address of the cable modem. Since I’ve given it the MAC address of the LAN card that connected successfully and it still couldn’t connect, I’m pretty sure it’s not a MAC address problem.

I’ve tried it with no DNS specified in the D-Link’s WAN menu, and I’ve tried it with the DNS that Comcast automatically assigns me. Since there’s no IP address being assigned to the D-Link, I’m not sure why the DNS would matter. My question about the subnets was whether I could have the 255.255.255.0 subnet both within my LAN and facing outward to the cable modem.

5

While both of the previous responses are accurate, I’m not sure you will have found them helpful.

Your problem is not subnetting. You should continue to see 255.255.255.0 as the subnet mask for your internal network - no worries there.

Chances are, DevNull is correct. The cable companies are worried with people buying cable modems are stealing high speed Internet from them. So they setup a system where their DHCP servers only hand out IP addresses to registered customers. The cable modem doesn’t have an IP address, and it doesn’t hand out IP addresses. In that sense, it is dumb. It is the cable companies routers and servers that hand out IP addresses.

The reason your computer gets an address when connected to the cable modem directly is because when you first setup your cable modem service (either manually or through some setup software you ran on your computer), the cable company learned the MAC address of the ethernet card in your computer. When your DLink router tries to get an IP address from the cable network, the cable network doesn’t recognize its MAC address, and refuses to give it an IP address.

There are two solutions here. One is to call the cable company and change the MAC address they have on your account. Since calling the cable company is very painful, I recommend the second solution.

The second solution is to leverage the “MAC Cloning” capability of the DLink router. I suspect you saw the option in the administration of the router. When you have your computer connected directly to the cable modem, and have received a valid Internet address, do a “IPCONFIG /ALL”. Look at the your ethernet interface, and jot down the MAC address (listed as “Physical Address”). It will be twelve hexidecimal digits, listed in six pairs separated by dashes. You can ignore the dashes.

Now hook your cable modem into the WAN interface of the DLink, and connect your computer to LAN side of the DLink (or connect wirelessly). Go into the administration of the DLink, go to the MAC Cloning section, and enter your ethernet card’s MAC address into the DLink. After you do that, reboot the router, and viola, it will work.

Now, when the DLink tries to get an IP address from the cable network, it pretends to be your registered ethernet card, the cable company is fooled, and hands it the address it otherwise would have handed your computer.

6

Have you checked the DI-624’s log for clues? Some cable modems have logging capability too. Mine does. Five lines. Cheapskates.

And of course the a hard reset shouldn’t hurt things but I bet you have done that a few times already.

7

The DLink site shows five revisions of the DI-624. If you post which one you have, I’ll look at the manual to tell you the settings that will work. Your problem isn’t DNS nor subnetting.

8

Everything seems to be working… just… fine. I did a firmware update, and then repeated my troubleshooting steps. Nothing worked, and then suddenly everything worked fine. The MAC address of the router is currently set to emulate the LAN card in my computer, and Bob’s your uncle. They have no good way of knowing what that MAC address is, though – yesterday when they asked I gave them the cable modem’s built-in MAC address and the D-Link’s address as a backup. They seem to have settled on this LAN card address as the Gatekeeper and/or Keymaster though. Just Gozer to show you can’t trust 'em.

9

Comcast doesn’t bind only registered MACs, or at least didn’t when I worked for them. They would only register one IP per modem session, meaning I got to tell people that if they wanted to use their XBox online, they’d have to unplug their modem, switch things around, then plug it back in.

Cloning your working desktop MAC should theoretically avoid the above requirement, but I get the feeling the WAN side of your router isn’t working so great.

I’d try unplugging your modem, resetting your router to factory default, then plugging the modem back in, then the router. If the router still doesn’t pull an IP, there’s probably a problem with it.