You're stepping into a mine field here!
You bet!
Unfortunaelty, I've yet to reinstall skype after the crash a few weeks back.
OK - my public IP address is dynamic (and that's fine - I understand this). But the issue seems to be that my public IP address is that of my DSL modem.
The DSL modem is connected to the Router which has its own private IP address (it also issues a static IP to the observatory PC). The observatory PC is setup in port-forwarding.
What seems to be confusing matters is my provider assigned IP address is assigned to the DSL modem (currently 78.16.26.12) - but the person who wants to connect to the observatory needs the router's ip address - right? or wrong?
Is the fact that I have a DSL moem conncted to a router complicating things? If so, what can I do to fix it (buy a better modem/router?)
Cheers
Dave
Hi Dave,
What do you mean by "router" and what do you mean by "modem", are they the same device?
The normal situation is that your modem and router are one box. Like all routers this one box has two IP addresses, an outside address (your public one), and an inside address, also a private IP (usually 192.168.1.1).
What happens inside this magic box is that between the internal and the external interface Network Address Translation happens. This means that all packets destined for the outside world get stripped open, get their private address removed and the router's public address stuck in instead. In effect all machines inside your network appear to be your public address. This is fine for connections initiated from inside the LAN, this can't work for connections coming in from outside. "I have a packet for port 80 on some machine ... aaaah .... which one?" That's where port-forwarding comes in. It tells NAT that an incoming connection on port X should be translated to Y private IP address.
However, some protocols are wrapped inside TCP-IP. When NAT does network address translation it ONLY translated the outter TCP-IP layer, not the wrapped protocol. So, the packet arrives at the right machine, the TCP/IP stuff gets stripped off and then the result gets handed to the program in question. If the program then checks the destination address against it's address it will see a miss-match and will probably throw away the packet. Protocols like this are said not to be able to traverse a NAT router. If this is the case here you are hosed. Nothing you can do.
Were I to be you I'd start with a very simple test. I's install an apache web server on the observatory machine, make it run on port 8080, port-forward port 8080 to that mahcine and then get a friend from outside to see if they can hit your web server. If they can your setup is right. If they can't it's not. You won't be able to test this from inside in all possability because you are on the wrong side of the NAT router so routing funnyness will probably occour as Al said.
Bart.
  