Why would a cable modem work in one location and not another?

Question

Up until last Thursday, my D-Link (don't know model) cable modem at home was working just fine. No issues besides needing a power cycle every once in awhile. Since Thursday, can no longer get online.

Borrowed a cable modem (same ISP) and can get online from home. Took MY cable modem to a friend's place approximately 10 blocks from my house. My cable modem works there, but not at my house.

Spent some time on Saturday debugging. Took the coax splitter out of the equation and am now connecting the modem directly to the primary coax cable. Also, took the router out of the equation and am directly connecting to the modem from my computer.

When Windows first starts up as I watch the network connection, it first says 'Invalid IP address' (Network Connections -> Status -> Support tab), then after a bit I get an "Automatic Private Address' starting with 169.XXX.XXX.XXX.

Got a tech coming by this afternoon, but curious if anyone has an idea what would cause my modem not to work in one location but work in another?

Solution 1

We tried several different computers, two different modems, directly connected to computer, connected through the router. The tech changed the connectors on the line that runs into the house. Nothing solved the issue.

What was really strange is that both modems report that they are able to connect but none of the computers could get an external IP address.

Ended up being some (unknown) issues on the line coming into the house from the tap. The tech had to run a new line into house.

Solution 2

This is highly dependent on your ISP.

Some ISPs tie down their connection to a specific MAC address and if it doesn't match up, you won't be able to connect or obtain an IP address.

Others have some sort of 'caching' going on. I have found some situations where I swapped out a router/modem and it does not work - much to my chagrin - however after letting it go 24 hours it starts to work. I can only assume there is something that prevents a new MAC address from working, maybe this is in an attempt to limit malicious users from swapping out a bunch of routers/modems to evade tracking, not sure.

Comments

[Admin]

Tried with router in place. Also with different computers. Same thing.

[Admin]

'Some ISPs tie down their connection to a specific MAC address' I don't think that's it as this modem has worked for quite some time before last week. 'however after letting it go 24 hours it starts to work.' Waited Thursday through Sat. morning.

[Admin]

well then, let's see what the 'Tech' has to say ... keep us posted :)

[Arjan]

But the question describes how switching the modems works, so using a modem at another location is not explicitly stopped by the ISP here.

[Arjan]

Odd, as earlier that friends modem did work, right? But well, good it's solved.

[mars]

Yeah, it worked directly connected but not when split. Apparently, the old line to the house was really pretty old.

[Nevin Williams]

Splitting the coax with a passive two-way splitter decreases the signal about 3 dB. This usually isn't a problem for your cable modem to receive a good signal, but if the cable modem already needs to transmit near its maximum level to reach the cable router, then the extra 3 dB of signal required might put the cable modem out of range. See dslreports.com/faq/5862 for more information.