Alphaplus

I am a beginner in the field and I don't know whether it is too silly to ask. What is the physical location(if any) of the four layers in TCP/IP model? Suppose I am using the application gmail, and has pressed the send button after typing in the mail when does application layer come into play and where is it located?Is it a part of the software and what of the other three layers?

Thanks. "Source code" was the word I was missing.

I am looking for the program. Not the name of it. It would be nice to see the program behind some app that is simple yet useful. (Also please help me figure out how to thank replies to queries on this forum).

Else how is it possible for google to display something asked in yahoo answers?