I think that the cable service is “keyed” to the MAC address of the cable modem, which then plugs into your NIC. Changing computers wouldn’t have an effect on this, as long as you kept the same cable modem. The network settings for the computer must still be set up to look to the cable modem to be assigned an IP address, etc. Likewise, putting in a router wouldn’t cause an issue because of the MAC address, as long as you knew what you were doing from a networking standpoint.
I’ve had AT&T @Home for over a year and a half. Within the past few months, I got a new puter. The old puter had ISA slots, thus an ISA NIC. New puter has no ISA slots whatsoever. Called up AT&T and they said I could have a tech come out and install a new NIC which would take a couple of days and cost some $$ for the service call, or I could go to Best Buy and buy a 10/100 Ethernet NIC. So I got back from Best Buy and had everything running just fine within a couple hours.
I had the old computer and was able to connect to AT&T’s customer service site which allows you to set up a chat session with a tech. MUCH faster than calling. If you have any problems with your setup in the future, and you can still log on, do that.
The cable has a limited bandwidth that is shared with everyone on your network (i.e. your neighbors). If a lot of your neighbors have AT&T and they are on at the same time (peak times), the throughput drops. With DSL, you have a dedicated line with a particular amount of bandwidth dedicated to you all the time. But during non-peak times, you get a much higher bandwidth with cable.
The IPs are dynamic. That is, you are assigned an IP address by the server, everytime you logon (or turn the cable modem on, in this case). This allows a company to use less IP addresses because not eveyone is logged in at the same time. People turn their cable modems off when they go to bed, for instance. A static IP address would be assigned to the cable modem and the modem uses that same address everytime it logs on. However, this does not allow for IP reuse.
Got two computers? Get one cable modem and an ethernet hub. The hub should have a port you can assign as uplink. Attach the uplink port to the modem and your computers to the other ports. You need to let AT&T know you have two computers so they can assign different names to the computers and thus get different IP addresses, otherwise you get all sorts of errors. Not sure if there is an additional charge for the second computer, I sold my old one before I got a chance to really play with the network configuration.
Hope this isn’t too much info. This is my first post to the SDMB. I’m a bit excited. Please excuse any spelling/grammatical errors. I’m an engineer, not an English teacher.
Welcome to the Straight Dope!
Be sure to ask him what’s it’s like being a minion of the evil intergalactic empire. 
To answer two questions that go together: a ‘static IP’ means that the number that identifies your computer on the 'net (so that when you click a hyperlink to view a new website, the host machine knows where to address the HTML data packet) is always the same. When you use a dialup internet service, you have a ‘dynamic IP’ address-a new number assigned every time you dial in. Usually, on a cable modem or DSL, you’ll have a static IP. And this means you will definitely want to have a firewall installed–otherwise, every script kiddie on the 'net will probably have the run of your computer within a week. Firewalls can be either software or hardware-software solutions are most appropriate for a home system. And the service provider may offer one for free or on the cheap as customer service-they’re providing the connection that lets those hackers into your private files, after all, and if that means you end up as a host for a ‘Code Red’ worm, well, that sucks for them almost as much as for you …
Another newbie! Welcome!
I am also called an engineer so I’m unfamiliar with the concept “too much info.” Thanks! Near as I can tell adding the second 'puter would cost $5/mo.
It has been mentioned before, but you could get a router - it costs more up front, but you wouldn’t have to pay the $5 per month. Plus it adds a level of firewall protection. this is how it works (the quick, sketchy version):
- The AT&T assigns their Dynamic IP address to the router instead of the computer. AT&T doesn’t have to know that it is giving the address to a router instead of the computer.
- The router assigns different non-routable IP address to the computers, or the computers have their own static non-routable ip addresses that the router can see. Non routable means that those ip addresses are reserved for use on the internal network and the router will do translation for them to get to the outside world, but never allow direct access to them.
- The router translates requests from the computers(inside the firewall - safer) and requests information from AT&T(outside the firewall-not so safe) on their behalf. When it gets the information that was requested, it remembers which computer requested what info, and “routes” it to the correct computer based on IP address.
It is much simpler than it sounds. I hear that Linksys has good low end routers that are easy to set up.
Or , if one of the computers is running Windows 98SE/ME/2000, you can get an extra NIC for it and use the built in Internet Connection Sharing - kind of clunky and has drawbacks, but a cheap NIC is $15-$20, so the price may be right.
The Linksys BEFSR41 has 4 ports built into it on the Intranet side, is a full-fledged router and switch, can use PPPoE if your system demands it (and makes it painless too), can masquerade it’s MAC address, can masquerade the computer Network ID (required by some systems, like Comcast), does have a superior firewall, doesn’t eat up enormous amounts of memory like ZoneAlarm, does work reliably with VPN (unlike ZoneAlarm), will let you use 4 PCs simultaneously with no charges per month…and is cool, to boot.
Don’t get me wrong - ZoneAlarm is overall a good, solid product (but it does seem to eat a lot of memory, especially if you leave your PC on all the time, like I do). But there are so many other advantages to using the $99 (with free shipping, from Amazon) Linksys that I really think you should consider it.
'Puter Dude here speaks highly of the Linksys hub and it’s the only firewall he uses at home because it does that masquerade stuff. Since my only concern with security is to keep joyriders out (it’s not like I keep anything important on my PC) it sounds like the equivalent to using a Club and locking the car doors on my rusty Tempo. Pros can get past it, but why would they bother? (FTR, I don’t even bother with The Club)
And adding another couple “terminals” without any fanciness (asking permission and paying extra) would be nice. And a guy here has a drill bit on a stiff cable so running cable would be easy (advantage to working here is that we do that stuff).
Are you suggesting my daughter’s spiteful ex-boyfriend might do something like that? Okay, point taken. 
But I’ve always been more likely to be screwed more by people trying to do me a favor than those wishing me ill. In both senses of the word.
Y’all want me to post the list of questions I assembled for the next time somebody asks this?