Apple does not officially support Port Forwarding (or Port Mapping as Apple calls it), so you won't find much about this in Apple Support documents.
If you need to forward a consecutive range of ports, it would look something like this: 8800-8888
If you need to forward consecutive ranges of ports, it would look something like this: 8800-8888, 9000-9200
If you need to forward non-consecutive ports, it would look something like this: 443, 445, 449, 555
The trick is going to be figuring out where to place the port values on an Apple router, so you will need to know more than just the port values. Apple wants to know whether the port value(s) will be allocated to a:
Public UDP Port
Public TCP Port
Private UDP Port
Private TCP Port
If you are using an Apple router, it would really help to know what the make and model number of the device that you call your "modem" might be. We ask this because most users refer to products like a modem/router or gateway as a "modem", and it's not.
And, if your "modem" is really a modem/router or gateway, you cannot set up port forwarding or port mapping on the Apple router, it must be set up on the modem/router or gateway while the AirPort is set up in what Apple calls "Off (Bridge Mode)".
To make things more challenging, port checking utilities are often wrong. They will show a port as "closed" when it is really "open". For example, to access the hard drive on my Time Capsule, Public TCP Port and Private TCP Port 445 must be "open". Yet, every "port checker" that I've ever tried shows the port as "closed" even when I know it is open and working correctly.
The bottom line here.......an Apple router is not the ideal product to use if you need to forward ports, since Apple does not support UPnP, a feature on most other routers that automatically opens the ports that are required for a access to a device on the network.