Apple doet het als volgt. Je hebt een zogenaamd Base Station:

this one is connected to phone/cambel modem or whatever connection

In the mac that needs an connection (to the LAN or internet) you put an Airport Card:

which goes in quite simply by the way :)

And than you've configure it (took me about 8 minutes). And you get something like this:

(play Apple's 'Airport game')

More specs about Airport in this PDF file