<HTML ><HEAD ><TITLE >Theory</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="VPN HOWTO" HREF="index.html"><LINK REL="PREVIOUS" TITLE="Related Documents" HREF="x174.html"><LINK REL="NEXT" TITLE="What is a VPN?" HREF="x189.html"></HEAD ><BODY CLASS="chapter" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >VPN HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="x174.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="x189.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="chapter" ><H1 ><A NAME="theory">Chapter 2. Theory</H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT >2.1. <A HREF="x189.html" >What is a VPN?</A ></DT ><DT >2.2. <A HREF="x192.html" >But really, what IS a VPN?</A ></DT ><DT >2.3. <A HREF="x195.html" >So how does it work?</A ></DT ><DT >2.4. <A HREF="x205.html" >SSH and PPP</A ></DT ><DT >2.5. <A HREF="x209.html" >Alternative VPN Systems</A ></DT ><DD ><DL ><DT >2.5.1. <A HREF="x209.html#AEN212" >PPTP</A ></DT ><DT >2.5.2. <A HREF="x209.html#AEN216" >IP Sec</A ></DT ><DT >2.5.3. <A HREF="x209.html#AEN220" >CIPE</A ></DT ></DL ></DD ></DL ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="x174.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="x189.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Related Documents</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >What is a VPN?</TD ></TR ></TABLE ></DIV ></BODY ></HTML >