Router ports for two NAS

[briareos_kerensky]

I have two NAS, one of them set up for remote operations: FTP, web interface, and so on. I've set up the router to direct incoming traffic from ports 8080 and 21 to the first NAS' LAN IP.
I'd like to be able to access the second NAS remotely as well (just the web interface, no need for FTP), but I'm not all too familiar with communication ports: if I were to dedicate a port to the second NAS, can I pick one at random (let's say 8081) or are some of them reserved for specific purposes?

[smouty]

In your router you should be able to have a random open port e.g. 12345 and redirect this to a standard port 22/80/8080 etc. internally.
I do this so I don't have standard ports exposed to the internet.

[briareos_kerensky]

OK, let's say I open port 123 and have the router redirect it to 8080, this takes care of one device.
I open port 456 for the second device, where the router should redirect? 8081, or some other port because the 8081 is reserved by something? If I understand correctly I need two ports open to manage two separate devices, with or without standard ports directly open to the net.

[smouty]

Your internal ports don't need to change and each external port is opened against a specific IP so multiple instance of the same port can exist over multiple devices e.g. for two web servers

Device 1 = www.yourdomain:123
internet -> 123 ->router -> 192.168.0.10 port 80

Device 2 = www.yourdomain:456
internet -> 456 ->router -> 192.168.0.11 port 80

An example shown in the image below, Port 8888 is translated to port 80 internally

[briareos_kerensky]

I see, it makes sense. I thought that each device should have had its own internal port.