Server

This section tells you how to set up the server side of things.  I figured that this should go first since without a server, your client is kind of useless.

Security − keeping people out
Security is very important for a VPN.  That's why you're building one in the first place, isn't it?  You need to keep a few things in mind while setting up your server.

What is a VPN?

What is a VPN?
VPN stands for Virtual Private Network.  A VPN uses the Internet as it's transport mechanism, while maintaining the security of the data on the VPN.

But really, what IS a VPN?
There are several answers to that question.  It really depends on your network layout.  The most common configuration is to have a single main internal network with remote nodes using VPN to gain full access to the central net.  The remote nodes are commonly remote offices or employees working from home.  You can also link two small (or  large) networks to form an even larger single network.

the ISDN transport

Work on developing ISDN began in the  1970s but  would  not  be sold  to the  bandwidth-hungry customer until the  early  to mid 1990s. Many said it was too  little too  late, and  the  consumer market for ISDN never took  off. After the  news  of the  first VoIP telephony call over  the Internet spread in 1995, a renewed interest in ISDN emerged for a short while. But by this  time our attention was turned to the  emerging DSL technology first deployed in 1998.

The eventual ISDN standard provided for two flavors of ISDN: Basic Rate Interface (BRI) and  Primary Rate Interface (PRI). The ISDN standard defines the  basic unit of bandwidth as a B channel, which provides 64 Kbps of band-  width. B stands for bearer channel, which is another name for the  channel that carries POTS calls over  the  PSTN.