Describe URL and hostname part?

The hostname part of the URL should be a valid Internet hostname. It can also be an IP address like as 204.29.207.217 or 128.235.252.184.

The port number is optional. It's not essential if the service is running on the default port, 80 for http servers.

The path points to a particular directory on the exactly server. The path is associative to the document root of the server, not of necessity to the root of the file system on the server. In general a server does not show its entire file system to clients. Indeed it might not really expose a file system at all. Rather it shows only the contents of a specified directory. This directory is known the server root, and all paths and filenames are associative to it. Therefore on a Unix workstation all files in which are available to the public might be in /var/public/html, but to somebody linked from a remote machine this directory looks like the root of the file system.

The filename points to a particular file in the directory specified by the path. It is frequent omitted in that case it is left to the server's discretion what file, if any, to send. Many servers will send an index file for that directory, frequent known as index.html. Others will send a list of the files in the directory. Others may send an error message.

The fragment identifier is used to reference a named anchor or ID in an HTML document. A named anchor is shaped in HTML document with an A element with a NAME attribute like this one:

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In more modern browsers we can also use an ID attribute on any element instead:

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