Jonathan Hutchins wrote:
To follow up: while ethernet is great for multiple hosts on a local loop or star topology within a building, it's not designed for long distances. You can often get away with a run between buildings, but when you start building cross-town links you need a different standard. That's where other protocols like Frame Relay and "DSL" come in. This is why you need some sort of "modem" between the cross-town link and your ethernet.
Unless you're using 1000BASE-ZX (100km):
http://wiki.ethereal.com/EthernetHardware
Or Long-Reach Ethernet (5000 ft):
http://www.cisco.com/warp/public/779/servpro/solutions/long_ethernet/
Or GigaMAN (180 mi):
http://www01.sbc.com/Products_Services/Business/ProdInfo_1/1,,1545--4-1-0,00...
There's nothing inherent in Ethernet that keeps you from carrying frames over long distances. Most cable and DSL connections simply bridge Ethernet frames from the home to the CO or head end, which often qualifies as cross-town.