The standard A-record (domain name to IP address) currently used by web-browsers to locate web-servers does not include a port number.
Current web-browsers use the URL protocol prefix (http://) to determine the port number (http = 80, https = 443, ftp = 21, etc.) unless the port number is specifically typed in the URL (for example "http://www.xpertdns.com:5000" = port 5000).
A recent addition to the DNS protocols - the SRV record type (RFC2782), does have the potential to solve this issue.
Future web-browser software may use this to determine the port number, but this is not yet supported by any of the major browsers.
For the time being, if you need to run a web-server on a TCP/IP port other than 80, the visitor will need to specify this directly in the URL, or you need to use a HTTP redirect (from a web-server on port 80).
For more information, please see the following knowledge base article:
KB: Can I use web redirection and "cloaking"?