A standard protocol that provides authentication and encryption for communications over the Internet. IPSec is used to secure VPN tunnel. Also see VPN, L2F, PPTP, and L2TP.
Secure transmission over IP. IPsec is an extension of the IP protocol used for encryption and authentication. Encryption occurs at the transport layer of the OSI model, the application doesn't have to support encryption for the encryption process to work. Therefore, all network traffic generated by applications can be encrypted regardless of the application See Also http://www.netbsd.org/Documentation/network/ipsec/.
See: Internet Protocol security