Now looking at the brower URL, http://www.google.com for example. The "www.google.com" part is basically an IP address so it is in the network layer. The "http" part is the application layer. So these two are the bricks of this whole URL thing that the brower needs. Of course, the brower needs a port number. But it can just use the default number 80.
One interesting thing I read about the linux "nc" command is that you can submit an email manually like the following:
$ nc localhost 25 << EOF HELO host.example.com MAIL FROM:So you just need to use the correct format of a given protocol on the designated port to use the service that uses the protocol.
RCPT TO: DATA Body of email. . QUIT EOF