20.4. Protecting VPNs with IP Security (IPSec)

Basic garden-variety IP lacks security, and that's fine in most cases—if you're just accessing files on a file server within your network, then you don't expect someone to be snooping. However, it is possible for someone between your computer and the server to intercept the data with a protocol analyzer (often called a sniffer). There's even the possibility for malicious users to modify the data in transit.

IPSec provides a generic solution for securing IP-based networks. IPSec operates at the same layer as IP and is a necessary part of using the VPN protocol L2TP—you can't run L2TP without IPSec. IPSec is supported by both IPv4 and IPv6.