Connecting to SQL 2005 Instance using an external IP address without specifying a port number

I have a Win2k3 Std 64bit server running SQL 2005 Std. It's internal IP address is mapped to an external IP address using NAT. External users need to connect to the SQL instance without specifiying a port number  (i.e. 'instance','port number') using the external IP address. At the moment users need to specify both the instance and port number to be able to connect to the database using the external IP. Is there any way of configuring SQL or the network to prevent having to use a port number to connect to the instance using the external IP address? Thanks in advance!
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I may be wrong but I thought that if the SQL Browser was running that it would handle it. You might have the browser turned off though...

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ccarrackAuthor Commented:
Thanks but the SQL Browser service is started.

Let me provide some more information. The server has a signle NIC assined with an inetrnal IP address . I have a Cisco router that handles the NAT statement - ip nat inside source static yyy.yyy.yyy.yyy . This allows the server to communicate with the outside world and vice versa. Ports 1433 and 1434 are both open on the firewall.

Users connect to the SQL database from the outside using the external address yyy.yyy.yyy.yyy . However when defining the instance value they need to enter 'instance_name,portnumber' and not just 'instance_name' which is what is required. If I do a netstat query I get the following information.
When no one is connected (or no connection can be made becuase user is using only the 'instance_name'):
TCP               LISTENING

When someone connects using  
TCP               LISTENING
TCP        zzz.zzz.zzz.zzz:1365      ESTABLISHED
where zzz.zzz.zzz.zzz

In SQL configuration manager I can only configure TCP/IP port numbers for the internal IP address (since there is only 1 NIC).

I think this is problem lies in the router configuration. How can I tell a Cisco router to route anything looking for yyy.yyy.yyy.yyy on a specific port? In this case port 1433.

Thanks again.
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