What type of modem do I need?

bridge modem
The 2100 modem is a bridge modem only, it doesn't do either NAT or PPPoE. Don't worry what that means, just that it's good news. It is already a dumb plain pass-through, so you won't have to dumb it down so that it won't conflict with the router's functions. I am also going to make a wild guess here that you are in a part of the Verizon network that was GTE before it was bought by Verizon, and are not using PPPoE, but rather DHCP. If you were in an area that was BellAtlantic before Verizon that means you are on PPPoE and you have had your Verizon DSL for 3 or 4 years or longer (about then they started using the 2200 modem for PPPoE customers).
Configure your Windows XP PPPoE client as follows:
1) Go to Start >Control Panel >Network and Internet Connections.
2) Select Network Connections under the Pick a Control Panel icon.
3) Click Create a new connection under Network tasks.
4) Enter your area code in the location information (if asked), and click OK twice.
5) Click Next when the "Welcome to the new connection wizard" window displays.
6) Select Connect to the Internet and click Next.
7) Select Set up my connection manually and click Next.
8) Select Connect using a broadband connection that requires a user name and password and click Next.
9) Type "DSL Modem? ( you can use your ISP Name here) for the ISP name, and click Next.
10) Enter your BellSouth username followed by "@ISP NAME (if required by your ISP)" and password. Confirm your password, and click Next.
11) Click Add shortcut to this connection to my desktop and the DSL Modem icon will be added to your desktop. Click Finish.
12) Click the DSL Modem icon to begin surfing.
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Best of luck