If you are using a traditional modem-based remote access service for users and administrators to access your network...
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from a remote location, you may want to take advantage of one of the features of the point-to-point protocol (PPP). This feature, known in Windows as 'callback,' has two major advantages.
Normally, clients simply call the remote access server via modem and an analog or ISDN phone line, are authenticated and gain access to the network. With callback, clients call the server, are authenticated, and then the server hangs up and calls the client back. Most of the time, administrators use this feature because it simplifies billing for long distance services. Clients have to pay for a phone call that typically lasts only a few seconds, then the server calls back, incurring the bulk of the charges. This makes for happy users and organizations can sometimes take advantage of price breaks.
The other reason it's used is for security. As we all know, war-dialing is still a problem even though it doesn't get the media coverage that credit card theft and Web site defacements get, and if someone finds your modem bank, the only thing separating that person from your network is a password on your RAS server. Now, if you configure callback so that it requires calls to be placed from specific numbers, then the PPP protocol prevents the network connection from being established before Windows security becomes an issue. The downside of course, is that you have to maintain an essentially static database of users and their (presumably) home phone numbers. This can be a hassle but sometimes the security is worth it.
Callback is configured in "Network and Dial-up Connections" and you can find configuration instructions and details in the online Help.
About the author
Thomas Alexander Lancaster IV is a consultant and author with over ten years of experience in the networking industry, focused on Internet infrastructure.