How to get the External IP Address with in the LAN?

I want to get the External IPAddress of the client system in the LAN. My server is their in LAN so nd I make a request to the s开发者_运维知识库erver through my local systems at the time I need that local system External IP Address through ASP.NET but it is giving the local IPAddress.

You can't do this programmatically without visiting some external tool, or getting support from your router. ghostdog has a good suggestion.