Setup ad hoc network for Windows 7 or Macs

If you have wireless adapter in your computers, you can setup  computer to computer network using ad hoc network without using router. The wireless signal will be created by one computer and received by another computer, without using external wireless. This is useful when you are in travel or you have only one external  network  wire plug in.
In Windows 7,
start->Control Panel-> Network Sharing Center->Setup new connection or network -> Set up a wireless ad hoc (computer-to-computer) network , click next

In Mac:
 Airport- or click wifi icon>Create Network->Create computer-to-computer network.

Self assign IP: 169.254.*.*