Virtual Private Networks, Second Edition

How the AltaVista Tunnel Works

Each AltaVista Tunnel network consists of two sides, the inbound and outbound, both connected to the Internet in some fashion. The inbound side is the private network, which consists of some number of hosts and an AltaVista Extranet server. The outbound side can be either a single computer or another LAN. In the first case, the user would run the AltaVista Telecommuter client, and in the second, the remote LAN would have an Extranet server of its own to manage inbound or outbound tunnel traffic by hosts on its network. The Extranet server on the inbound side always manages authentication, dynamic IP assignment, and the routing of tunnel traffic for incoming connections.

A simple VPN might consist of three individual users connected to the Internet via three separate ISPs. Each user is running a Telecommuter client, and has access to the tunnel group through the remote LAN’s Extranet server. Users have unique physical IP addresses on their respective ISPs’ networks.

The tunnel server and hosts on the remote LAN are likewise assigned physical IP addresses. On the tunnel server, a range of virtual IP addresses is available for assignment to incoming tunnel connections. Each tunnel receives two virtual IP addresses: one for the client end and one for the server end.

Tunnel traffic destined for the virtual private network is first routed, via the tunnel client software, from the client’s physical IP address to its virtual IP address. This virtual IP address ...