What's the difference between an Ethernet ADSL Modem and an Ethernet ADSL Modem Router?
An ethernet ADSL Modem will pass on the IP address assigned to it to the device attached (whether this be a computer or Airport Extreme Base Station or Airport Express). In this case the AEBS would then distribute IP addresses to devices on a network.
Whilst an ADSL Modem Router will (using
NAT (and DHCP)) assign IP addresses to devices on the network, in which case
the Airport Extreme Base Station or Airport Express should be configured as
a wireless access point.
Each device has its own advantages and disadvantages, a lot will depend on whether you want to do more with your connection than simply surf the internet, ie host games, run web servers, and so on...
Some ADSL Modem Routers have highly configurable firewalls compared to an AEBS which has a simple NAT firewall which can only open a certain number of ports.
Note that some manufacturers do not support Macs, which means that the ADSL Modem Routers work fine and are configured by a browser, but if you try and get help and support from them they may not be able to.
In most cases, most people do not need a highly configurable firewall and the AEBS will suffice.