ESP8266 Community Forum

I'd suggest you read up on NAT in routers and why port forwarding is used to provide external access. That will allow you to understand why when a router gets a random packet from the outside world addressed to its one IP external router address it would have no idea what to with it.

Basically the only way communication to a device behind NAT can be happen is either it is initiated by that device or a port forward rule is established on the NAT router.

Initiation by the device can avoid the need for port forwarding but implies that there is either a permanent connection or a periodic reconnect / polling mechanism in place.

and forget the idea that in any serious company with an it security scheme worth mentioning they will allow/have uPNP active on their router/firewall.

Reverse connection, Turning a server into a client, on a Windows platform. Executable and source included at bottom.

http://www.flypig.co.uk/?page=sercli&dnload=beacon

The role reversal is a useful concept, but it still requires the device behind the NAT to initiate the transaction. In this case the beacon is making periodic connection attempts (polling).

The OP seemed to be seeking a solution where the external device directly addresses the internal device without any delay or action on the internal devices part and without setting something up specifically on the NAT router. Once it is realised that this is not achievable then a number of other methods can probably be used to actually deliver a feasible solution.