Hi alaios,
well then...
the first tricky detail is, that the IP-Address is an internal IP. (I think, you know this, it's the same problem as nat)
the second is, that the browser has first to send it's requests to the proxy. The proxy won't reply if it is not his own IP or name.
This would be done, if you use net. So you have to send a request that is looking like this:
CONNECT
http://www.xxx.yyy/zzz.html
Then the proxy GETs the page and sends it back to the browser.
This has to be done for every object you want to get over the HTTP protocol.
This also means that the proxy is able to cache the requests, nat isn't able, because it only knows the IP, not the full "path".
So, it is practice to not listen on port 80, because http and "proxy" is different.
The proxy will also get an address like this for you.
http://testserver.org:1234/
This is also http but it uses an other port.
Hope this is what you want to hear ;-)
cu doehn