What IP is calling my web method?

I have a web service which are called by some clients. I would like to be able to log what IP the clients are calling from. I cannot change the interface of the methods. I am aware of IIS logs but I need to be able to log the IP's programmatically as the web service is being called.

I have seen: How to identify who is calling my web services? and Who is calling my WebService? but they do not provide the solution I am looking for.

Thank you.