Question : Problem: Port forwarding - one port to many based on url

Situation:
Our office is connected to the Internet with high speed connection. Our ISP opened 5 ports for us. Their CISCO router forwards SMTP, POP3 and HTTP ports to Debian etch web/mail server in our local network. RDP and Kerio Winroute firewall VPN (port 4090) ports are forwarded to another WIn XP computer in our local network.

Problem:
We would like to use more ports. Extra block of 5 ports that our ISP forwards is too expensive. Is it possible to establish some sort of port forwarding/translation so that requests to specified port are forwarded to another port based on domain name or some other query?

Example:
ssh.ourcompany.com:4090  forwards request to   192.168.1.1:22
imap.ourcompany.com:4090  forwards request to   192.168.1.10:143

I thank you for your ingenious answers in advance :). Solutions in windows or linux are fine.

Answer : Problem: Port forwarding - one port to many based on url

You just need a SSH-Server in you inner network, and you only need the SSH-Port (22) to be opened.

The rest is freely configurable by you / your users. Teh connect to the SSH-Server with some defined Tunnels and connect only to Localhost:Defined-Port

localhost:2222 -> 192.168.1.10:143
localhost:2223 -> 192.168.1.20:80
....
...