IPSec NAT Transparency
The IPSec NAT Transparency feature introduces support for IP Security (IPSec) traffic to travel through Network Address Translation (NAT) or Port Address Translation (PAT) points in the network by addressing many known incompatibilities between NAT and IPSec.
Before the introduction of this feature, a standard IPSec virtual private network (VPN) tunnel would not work if there were one or more NAT or PAT points in the delivery path of the IPSec packet. This feature makes NAT IPSec-aware, thereby, allowing remote access users to build IPSec tunnels to home gateways.