Your ISP will have given the same ip you have to many customers, therefore it's not a good idea to use common known ports. You must generally not use ports below 1024.
You can configure RPC to use a different port using rpccfg.exe Here's an explanation & a recommendation about it.
http://support.microsoft.com/kb/908472[
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You may check common port number usage here
http://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers[
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Also note, that ports may be blocked by your router (or broadband modem). You must login to your router and forward the port numbers you use, so that other may reach your service at your port. (remember not to forward ports below 1024)
Just google for info on how to forward ports with your router/modem model no. I'm sure you'll find it
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