Actually you can have multiple IPSEC sessions hidden behind a NAT box with
a single public IP address - we've found several vendors' "routers" that
can work in this environment. I believe the key is that each tunnel must
be to distinct remote IP addresses. All the NAT box has available to
separate the traffic for the different tunnels (which use IP protocol 50)
is the address of the other end of the tunnel, but that is all it needs.

Of course, many users would like to have multiple tunnels to the same
partner. I don't know how that is possible with current IPSEC technology.

Tony Rall

I believe that at least one VPN client also does UDP encapsulation for IPSEC packets specifically for NAT traversal.


Cisco 5000 series VPN concentrators support a "NAT Transparency mode" which causes the IPsec packets to be wrapped in UDP and TCP traffic to port 80 at the concentrator. And this method certainly does support multiple IPsec tunnels from multiple clients, going to a single concentrator, all from a single public IP.

The Cisco VPN client does UDP encapsulation. But of course, the "server"
needs to support it as well. The PIX OS does not; the concentrators do.