Router - Port Forwarding

Router - Cisco Router

 You will need to have already set your interfaces to inside and outside using the commands,

ip nat ouside
ip nat inside 

Once done you can add the command below. This would allow the IP 10.8.8.8 to be accessed on port 23 via the IP address of the interface fa 0/0 using port 2333.

ip nat inside source static tcp 10.8.8.8 23 interface fastEthernet 0/0 2333

Note !! Rember that in the case above the syntax is for NAT`ing the source but also works the other way round, i.e NATs the destination.