IPSec, IP Security Protocol

IPSec is an emerging TCP/IP protocol that enhances TCP/IP by providing
authentication, using Kerberos V5 and encryption services, using Data
Encryption Standard (DES), Triple DES and 40-bit DES between computers
running IPSec. Currently, it is only available for Windows 2000 clients, not
for other Windows operating systems.

-> Step-by-Step Guide to Internet Protocol Security (IPSec)

This guide focuses on the fastest way to use IPSec transport mode to secure
application traffic between a client and a server. It demonstrates how to
enable security using IPSec default policies between two Windows 2000-based
systems that belong to a Windows 2000 domain. Once the two computers have
joined the domain, you should complete the first part of the walkthrough,
which demonstrates default policies in 30 minutes or less. Notes are
included on how to enable non-IPSec clients to communicate to the server.
Steps are provided on how to use certificates, and how to build your own
custom policy for further interoperability testing, or to demonstrate IPSec
when a Windows 2000 domain is not available.
source: ->Microsoft