9. Using IPsec to build VPNs

VPNs (Virtual Private Networks) were originally designed to facilitate communications between companies and their partners, internal communications within a multi-site enterprise, and even communications between company nomads and the parent network .

The technique used simply involves setting up an IP tunnel and exchanging data in the tunnel.

In concrete terms, an IP tunnel involves encapsulating an IP packet in another IP packet, so that two remote devices can exchange information...