This final Apple piece is rather specific. Basically, Mac OS X 10.4 (Tiger) broke compatibility with a number of routers. Once you install Tiger into a Mac, and plug that Mac into a router, no computer accessing the internet using that router will be able to access the internet. Disconnecting the Mac and resetting the router solves the problem.
There is a solution. (which might not be of universal application) Basically, you want to define your DNS servers. This is true whether you're using wireless, or an ethernet cable. If you're using a BT Voyager 2100, this fix should work, even though the BT site states that only Mac OS X 10.2 and below are supported.
You define your DNS servers in System Preferences | Network | *** | TCP/IP, where *** is your connection; you may need to reset the router afterwards.
The DNS servers for BT ADSL service are 194.72.9.34 and 62.6.40.178. If those servers do not work for you, you must look in your router's control panel to find out the correct DNS server addresses. For the BT Voyager 2100, look under Advanced | Status | Overview. You will need to know your router's access code; the default username and password for the BT Voyager 2100 are "admin" and "admin", respectively.
(NB: BT = British Telecoms. This advice was written for the UK.)
