Windows Help: Refreshing your network connection

Windows NT/2000/XP

go to Start -> Run ->
type 'cmd'; at the prompt type 'ipconfig /release', then 'ipconfig
/renew'.

Under Windows 95/98

go to Start -> Run -> type 'winipcfg';
hit the "release all" and "renew all" buttons.

On a Macintosh

go to the Network control panel; set it to be configured Manually, then set
it back to use DHCP.

Rebooting is another way to force the computer to try to get a new DHCP lease if the other methods don't work.