A friend asked me to set up a router to work with his Time Warner cable modem. The symptoms were confusing: if you plugged his PC into the cable modem directly, the cable modem would give it an IP address and everything was happy. If you plugged the router into the cable modem, there was a link light but nothing else.
After a couple unhelpful tech support calls, during which T-W's techs repeatedly told me I'd have to call the router manufacturer, despite my patient explanations that the router manufacturer wouldn't know diddly-squat about how T-W set up its network, a Google search got to the bottom of it: Time Warner is filtering by MAC address.
You see, every Ethernet device in the world has a unique serial number, called a MAC address. When the Time Warner tech set up my friend's cable connection, he must've configured the cable modem so it would answer to the Ethernet jack on the back of my friend's laptop--and only to that Ethernet jack. Because of the way T-W set up their cable modem, if my friend had to replace the laptop, or decided to plug in a desktop machine instead, or bought a PC-Card Ethernet card and plugged that in, or plugged in a router, the cable modem wouldn't deign to talk to it because it would have a different MAC address. Of course, none of the techs mentioned anything about this limitation when we talked to them, even after the Google search when my friend was pressing them about whether there was any sort of filter in the modem.
Anyway, the work-around is easy: newer routers, like the Linksys WRT54G, have a feature called "MAC address cloning." The router can change its Ethernet's MAC address to match whatever one your PC uses. Clone the MAC address and suddenly the cable modem is friends with your router.
Of course, it's up to you whether you want to do business with a company that goes to such lengths to make it hard to use the service it's purportedly selling you. Personally, I have DSL with one of T-W's competitors.