Problems with connection on sql express 2005 named instance over the internet

Hi,
Can You help me with connection to sql server 2005 named instance.
I have to redirect connection from public IP to local Ip of server with port 1433.
Local tcp/ip connection is  1xx.1xx.xx.xx\sqlexpress. or 'computername\sqlexpress'
On router I can only put  tcp/ip address without sqlexpress. 1xx.1xx.xx.xx.
Can You sugest me where I am wrong. What should I try do do.
Thanks
Zeljko

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