YES..you need to use the static NAT mapping (actually its static PAT too) Lets say the outside ip address is 1.1.1.1 and inside network is 172.16.0.0 ( .1 to .4) then in order to telnet from outside to inside privet ip address, the PC which is on the internet has fo fire request for telnet not on the port 23 for 1.1.1.1 but on different ports statically. That ports are mapped to the port 23 on the inside ip addresses..like this
ip nat inside source static tcp 172.16.0.1 23 1.1.1.1 1023 extendable
ip nat inside source static tcp 172.16.0.2 23 1.1.1.1 2023 extendable
so forth
So if the router see the request for tcp port 1023 on ip address 1.1.1.1 (outside), it will conver that to port 23 on inside ip address 172.16.0.1.
But for that client side has to initiate the telnet request for port 1023 on ip 1.1.1.1 to get to 172.16.0.1..
Hope this helps.