Are port forwarding settings saved on the Airport Extreme or the attached Mac

[Libertine Lush]

I need up figure out how to set up forwarding for a few programs I use, however, the Mac I'm using at the moment will be replaced soon with a new one. So I'd like to know if port forwarding settings are saved on the Airport Extreme or the Mac connected to it. If it's saved on the Mac, then I would just wait till I get my replacement to set up port forwarding.

Thanks for any help.

 

[Makosuke]

They're on the Airport itself, as is everything you set up with the Airport Utility.

The whole point of port forwarding is to have the Airport redirect a connection to a specific computer before it gets to the internal network, so it pretty much has to be on the router itself.

Note that unless you set the airport to give your replacement Mac the same IP address as the current one, you're going to need to adjust the port forwarding settings later, anyway. It's pretty easy to do, though, so I don't see any reason not to set it up now.

Also note that at least newer Airport base stations support UPnP, so if your internal program supports that you can just turn it on and it will auto-configure. Transmission, for example, has this feature, and it works great.

 

[Libertine Lush]

Note that unless you set the airport to give your replacement Mac the same IP address as the current one, you're going to need to adjust the port forwarding settings later, anyway.

So unless I manually adjust the IP address of the new iMac, it will be different than the old iMac? OK, makes good sense, even to someone less tech savy. I just seem to recall there's an external IP and an internal IP, so I assume the IP of the computer must be the internal IP.

And you can in fact change the IP of a computer? I thought that was only something possible through the ISP, ie dynamic vs static IP. But perhaps I'm confusing internal and external again.

Thank you.

 

[Makosuke]

You have no control over your external IP address--the one that the modem gives whatever is connected to itself (in this case the AEBS's external network connection). To computers outside your network, everything within appears to have this IP address.

The internal IP address of everything on your internal network, however, is set (usually) by the router; the whole point of port forwarding is so that when a computer outside your network needs to initiate a connection directly to a computer within, the router knows which specific internal computer to forward the connection to.

To do this, you create a port forward; you tell the router "whenever someone tries to initiate a connection to port XXXX from the outside, route that connection to port YYYY on IP address ###.###.###.### on the internal network." Obviously for this to function the way you want, you need to have a consistent IP address on the internal computer.

Since most home routers are set up to use DHCP (such that the router gives an available IP address to any connected computer that asks for one), the easiest way to do this is to go to the router's DHCP configuration section and tell it to always give one specific computer one specific IP address (you specify the computer using its MAC address, aka "hardware address", which is unique for every computer--your router can usually tell you what it is, or you can get it from the System Profiler). That way, the router will reserve that specific IP address for that specific computer, and give it to that computer whenever it connects.

In your case, when you replace your computer, all you need to do is change the MAC address in the DHCP section to match the new computer; the new one will then get the correct IP address and the port forward will work.

Again, with Apple hardware (maybe some other routers as well), you can also use UPnP if the software supports it; that essentially sends a signal to the router to automatically configure whatever port the software requests to forward to that specific computer, and it can be changed on the fly.

This stuff is kind of confusing, but it's not horribly complex to set up once you understand what you're trying to do.

 

[Libertine Lush]

Indeed it's confusing but your explanation is very informative. And educational. Thanks for all that. I do understand it all better now.

...the easiest way to do this is to go to the router's DHCP configuration section and tell it to always give one specific computer one specific IP address ...the router will reserve that specific IP address for that specific computer, and give it to that computer whenever it connects.

So is that say that normally, without first manually doing what you described, a router will assign a different internal IP address to each computer within the network every time each computer is turned off and on again?

 

[Makosuke]

So is that say that normally, without first manually doing what you described, a router will assign a different internal IP address to each computer within the network every time each computer is turned off and on again?

Yes. Or at least, possibly different. Usually they assign addresses starting at a number and incrementing up from that starting point by one each time another computer is given an address. So the first one would be, say, 192.168.0.1 followed by 192.168.0.2 and so on. They'll usually recycle lower numbers if that computer gets turned off/put to sleep for a long time.

If you always turned your computers on and off in the same order, the same one might usually end up with the same IP address, but there's no guarantee. Hence specifying a specific one, to be sure.

 

[Libertine Lush]

Ah, that's interesting. Thank you again.