need help on http error 404. the request resource is not found

Hi

I have three systems running on IIS7 with static Ip address 192.168.66.5. I have configired the system to run on the same Ip address with different ports and subdmain like;

1. 192.168.66.5:81 hostname ams.tpf.go.tz

2. 192.168.66.5:82 hostname gmis.tpf.go.tz

3. 192.168.66.5:83 hostname records.tpf.go.tz

4. 192.168.66.5:84 hostname cmis.tpf.go.tz

I configure all these on IIS7 and difined them in the router but i need help when I client types ams.tpf.go.tz should directly go to the system without specifying port number because if he doesnot put the port number sayhttp://ams.tpf.go.tz:81 it give me an errros http error 404. the request resource is not found