NAT-T Subnets
This option allows you to define a set of IPs (or entire networks) for your
clients to use when they establish an IPsec VPN. From these addresses the
client may create a "Virtual IP".
If you are not sure what any of this means then just leave the box empty
and make sure NAT Traversal is Disabled. Please refer to your EnGarde
documentation for more information on this subject.