networking problem

[D. Hugh Redelmeier]

| From: Zbigniew Koziol <softquake-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org>

| I will explain first in a not detailed manner. If someone thinks that a
| solution may exists, I may privately send more details.

[I'm not interested in private messages.]

| We have a connection from our office to a wire. A router is attached there.
| Our internal LAN works fine, we have internet, no problem with that.
| 
| We wanted to have access to one single computer that is inside of our local
| LAN from outside, from the internet.
| 
| We got a special IP for that, available form the internet. This is the point
| where I start to NOT understand things.

It sounds like you are behind NAPT.
<http://en.wikipedia.org/wiki/Network_address_translation>

The internet addresses nodes by IP addresses.  Most are globally
routable.  A few are not.  With NAPT, you can have a whole bunch of
nodes hiding behind one gateway with only the gateway having a
globally routable address.  The gateway implements the NAPT.

One important side-effect of NAPT is that (unless tricks are
performed), all communications between the internet and the LAN must
be initiated by the LAN side.

This is probably your problem: your home computer wants to initiate a
communication with a computer on your university LAN but the computer
at the university has no globally routable address.

There are many tricks to work around these NAPT problems:
- STUN
- NAT traversal
- port forwarding
- passive FTP
- UPnP
- IPv6

What IP addresses do your computers have on the LAN?  Are they in the
private address space?  Here is a list from RFC 1918:

     10.0.0.0        -   10.255.255.255  (10/8 prefix)
     172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
     192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

If so, that strongly suggests NAPT is your issue.
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