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What is a NAT Firewall?
Answer: A NAT Firewall isolates all of you computers from the
internet by stopping network traffic which has NOT originated from a
computer within your network. So in the following example
"Computer A" can talk to "Home Computer", but the fire
wall will block communication from "Home Computer" to
"Computer A".

So when using DIY Online Backup, for the "Home Computer" to
send backup files to "Computer A", the Network Traffic must be
allowed through the Office NAT Firewall. Of course at this point
we have kept everything simple, we have not talked about IP address and IP
Ports. So here we go....
IP Addresses - Each computer on the internet is given an IP address, so
when it requests data (Web Page, etc) other computers know who to send the
appropriate data back too. In the above example "Computer A"
and "Computer B" share the same IP address on the internet.
But "Computer A" and "Computer B" have IP address on the
internal "Office Network".

The routing or transmission between the "Office Network" and
the internet is taken care of by the Router, which is usually in the same
little balck box as the NAT Firewall (the one with the flashing
lights). So in the above example we need to program the NAT Firewall
to allow traffic from the outside world to "Computer A" (IP
192.168.1.10).
We now need to explain one more point!
We have seen from the example we want "Computer A" to
listen for communication from "Home computer" when DIY On-line
backup needs to write backup information to Computer A. So how does
DIY Online Backup know the traffic is for it?
Answer: The IP Addressing system also has the notion of
Ports. So when data is sent to a computer, the computer can route the
traffic/data to the correct program which is going to process it. Some
Posts are allocated for specific jobs, such as 80 is for web browsers, 110
and 20 for e-mail sending and receiving, and so on. By default On-Line
Backup will listen on port 2222 onwards.

So we need to tell the NAT Firewall to forward all IP
88.23.41.2:2222 traffic to IP 192.168.1.10:2222. DIY Online Backup
will give you the exact IP address if it failed to communicate with another
computer. These address are just an example.
How do I allow communication through the NAT Firewall?
The first problem we are faced with is that the interface to allow
traffic through a NAT firewall is different, depending on the
brand. Some NAT firewalls provide a simple utility, which is run
from the computer which requires the incoming traffic, in the above case
"Computer A".
DIY Online Backup will give you the exact IP address if it failed to
communicate with another computer.
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