What Happens If Protocol or Port Number Is Missing in a URL?

From a syntax point of view, all URL parts are optional. If some parts are not included in a URL, the browser and the hosting computer will provide default values. Here are some rules about the protocol name, host name, and port number:

1. If the protocol name part is missing, but the host name part is given, "http://" will be used as the default value. The following URLs are identical:

  dev.fyicenter.com/faq/xhtml/
  http://dev.fyicenter.com/faq/xhtml/

2. If the port number part is missing, but the host name part is given, the default port number that matches the protocol name will be used. The following URLs are identical:

  dev.fyicenter.com/faq/xhtml/
  dev.fyicenter.com:80/faq/xhtml/
  http://dev.fyicenter.com/faq/xhtml/
  http://dev.fyicenter.com:80/faq/xhtml/

3. If the port number part is given, the host name part must be specified. The following URLs are invalid:

  http://:8080/
  :80

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