Re: understanding nat

[Martin Mewes <macmewes gmx de>]

Hi Daniel,

daniel writes:

> now as i understand it, if my windows box is surfing the web, the
> packets sent out go through the gateway and have their source ports
> changed by nat so that when i get a response back, the gateway
> knows what box to send the data to right?

WindowsHost has: 192.168.1.35
Gateway has internal: 192.168.1.1
Gateway has external: <some public IP>

If your Windows Host is surfing he speaks with the internal Gatway 
which will transmit the wish to the Internet.
He will submit the wish with his external adress and knows what the 
internal adress was which originally submitted this.

If this is what you know, then you are right :-)

> but my question comes up when we're talking about the other box, my
> webserver that delivers several services through ports that can't
> change (22, 80, 25, 53, 110) when they leave... i think.

You speak of "Port Forwarding".

If me with some public IP wants to connect to your host, then it may 
looks like this ;-)

Me: Internet, please give me the WebSite "foo"
Internet: "foo" has IP-Adress "1.2.3.4 - please try again
Me: Host 1.2.3.4, please let me connect to "foo"
1.2.3.4: OK, here you are ...

What I do not know is, that 1.2.3.4 ist your external Gateway IP. He 
knows that if me ask for the WebSite "foo" that he has to connect to 
the internal WebHost on Port 80.

No the connection is established between Me and Foo :-)

####
Sorry if this sounds a little bit childish. I have to write a litle 
exam in this teaching children (users) on how internet functions ;-)
####

kind regards

MM

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