You can change the port number for telnet using simple IP NAT. Create a loopback on the device you want to change the port on and use this as the NAT inside. Use your LAN interface as the NAT outside and use TCP port translation. e.g R1 telnets to R2 using outside global address 99.99.99.99 port 99

R2

----

Interface Loop 0

ip address 2.2.2.2 255.255.255.255

ip nat inside

Interface fast 0/0 ;local LAN

ip address 192.168.10.10 255.255.255.0

ip nat outside

ip nat inside source static tcp 2.2.2.2 23 99.99.99.99 99

ensure R2 has a route to R1 and ensure R1 has a route to the inside global address 99.99.99.99 via R2.

From R1 you will be able to telnet via port 99 e.g.

telnet 99.99.99.99 99

Tie down all tcp port 23 traffic via access-lists. Almost a basic IOS firewall in reverse and works with most IOS.

Hope this helps.

Johnathan.

CCIE#9332