If you have a cable modem setup (such as @Home chello, etc.), you may or may not have trouble getting online in BeOS, depending on how they set you up (you must also have a BeOS-compatible network card, of course). First, try setting up your connection with DHCP enabled in Network prefs. Theoretically, that should work, though there are multiple flavors of DHCP, and not all of them work with BeOS.
If DHCP isn’t working for you under BeOS, try calling your ISP’s tech support and asking for a static address. You’ll need to obtain this information and enter it into Network preferences:
If they won’t give you a static address, you can grab your current dynamically assigned address from within Windows by opening a DOS window and typing:

IPCONFIG /ALL

Copy the IP address and use it in BeOS. It should be valid for a few days, until the DHCP lease expires. With some providers, the lease will expire after a day or two of non-use, so if you’re online every day, you may never have to reconfigure.