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Most Cable modems just bridge/convert your Ethernet (100baseT) into modulated Coaxial signaling. They have no "smarts". Therefore, for either the "protection of a NAT firewall" or to "share the broadband connection" with other computers Cable subscribers will insert a "router" in between the modem and thier computer(s).
Most ADSL modems, on the other hand have router functionality built right into the modem. Some have more "robust" router functionality than others, but generally they all to have at least a DHCP server and a NAT firewall. Note: this means that the public IP number will be consumed by the Modem under most configurations and all devices inside the network will get a private IP number.
Since an ADSL modem already has a DHCP server and a NAT firewall, you don't need to have a separate router. If there is an insufficient number of ethernet ports on the modem, all you need is a hub or a switch to give you more ports. If your ADSL modem doesn't support wireless connections, add a wireless AP.
If the router functions of the ADSL modem is insufficient for your network, you may want to continue to use the separate external router. The first caveat is that all DHCP servers being used will have to be configured to function on different network segments. See your documentation for more details. Since your traffic is then being routed through 2 NAT firewalls, you may want to consider doing one of the following adjustments to your network.
Option #1: Do nothing. Your traffic will have to traverse two separate NAT firewalls (one in the router and one in the modem.) Not a problem, but then whenever you want to forward a port, you will have to forward that port on both the router and the modem.
Option #2: Put the Router in the modem's DMZ. You'll still be doing double NAT and the external IP will still be consumed by the modem, but when forwarding ports, you'll just have to change the modem. see: »US West/Qwest DSL »How Do I Put A Router In My 701g's DMZ?
Option #3: Put the modem in bridging mode. (not supported or even available in all areas) see: »US West/Qwest DSL »How Do I Connect My 701 In Bridged Mode?
Option #4: use a half-bridging or DMZ+ capable modem. Note that the Qwest supported 2 wire modem does support this feature. See: »DHCP Spoofing / Half Bridging