Many times you would like to experiment with network settings and protocol when your system either doesn't have a network card, or isn't connected to the network. To do this, you use a function called the loopback adapter. The loopback adapter emulates your network, and returns responses to network commands. This function is also useful in determining if your protocols are properly set up if your networking card is malfunctioning.
To add the loopback adapter to Windows, open the Control Panel and double click on the Add/Remove Hardware wizard. The wizard will search your installed hardware and present you with a list, topped by the Add a new device entry. Click on that, and then in the network adapters list select the Microsoft in the left pane. Loopback Adapter will appear in the right pane, and you should select that.
The loopback adapter takes an IP address. When you enable DHCP the DHCP loopback adapter is assigned the address of 169.254.x.x/16 with a subnet mask of 255.255.0.0. That address indicates that it can't be part of any network.
To test the loopback adapter, open a command prompt, and use IPCONFIG to display all installed adapter addresses and then PING that address.
A tech note,
Barrie Sosinsky (firstname.lastname@example.org)is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.
This was first published in July 2002