I'm also using a cable modem, and this same thing confused me.
If you have your OS9 settings on DHCP, and you pull up your TCP/IP settings, you WILL see your IP address. However, this is the address which was assigned to you by the DHCP server. It is not necessary, (in fact, impossible) to put this address in manually when configuring OSX. Just select DHCP, and next time you reboot the server will assign you another IP address.
DHCP will not work in my OS X because I have to put in the cleint I.D. in order to connect to the DHCP server. When I choose DHCP all I get is bogus numbers that are so far from the correct domains I know something is not right. I've been told by @home (Cable Co) that my IP never changes so I just entered everything manually and it works fine - Apple will need to fix this in the final release or DHCP just won't work with anyone on a system such as mine. - john.
Look for Host name - everyone in the word except for apple referes to what AT&T is looking for as Host name, not client ID. With Windows and Linux systems, what they're really refering to is host name.
SO - with windows, when you're trying to connect, the AT&T guys will set you machine name (this sets your host name/DHCP client name automatically). If you were to want to have a different Windows "Machine Name" than host name/client ID, you go into network properties and change the information on the "Identification Tab". Then go into TCP/IP properties for your network adapter and change the "host" setting. It will warn you that they are different, but you'll still be able to use that kind of thing just fine.
Apple's "Client ID" is kindof an icorrect naming convention.
As for cable and their method of DHCP, it's not really DHCP anyway, since it's assinging your an IP address based on DNS information, and it acts weird anyway. But that's the answer, look for DNS Host name, and that should get you up and working.
That's the computer name. It's different than the client ID. It's not used for DHCP or TCP/IP in any way, shape, or form. If you go to TCP/IP properties, and click on the DHCP tab, the Client Name is what is really used for TCP/IP and DHCP stuff. The machine name could be "Fred", and as long as the Host name (in the TCP/IP properties) matched whatever your DHCP server needed to see, you'd be fine. The thing is, though, that the Machine name and the Host name usually match, and Windows will warn you if they don't, so it can appear that the machine name is used for DHCP configuration.
To clarify, this is how it works for Windows 95, 98, and NT 4.0. I have not really worked with any flavors of 2000 or Me, so I do not know if they've changed it.