From a post at DotNetNuke.com:
You'll need to include the
element < add key="UsePortNumber" value="True" / > in the
< appSettings > node of your web.config file. This flag will
enable the port-parsing code in the urlRewrite httpmodule.
Your final httpalias will look something like:
localhost:30000
You will also need to make sure that the port number is included in the portalalias table.