Common Internet Scheme Syntax

In theory, each scheme may use a completely different syntax entirely for the “<scheme-specific-part>”. However, many of these schemes share a common syntax for this part, by virtue of the similarities in how they refer to internetwork devices and resources on those devices. For example, both HTTP and FTP are used to point to specific TCP/IP devices using a DNS name or IP address, and then access resources stored in a hierarchical directory structure. It makes sense that their URLs would be at least somewhat similar.

The most general form of this common Internet scheme syntax is as follows:

<scheme>://<user>:<password>@<host>:<port>/<url-path>;<params>?<query>#<fragment>

The elements of this syntax are as follows:

• <scheme>: The URL scheme, as described above.
  
  
• <user> and <password>: Authentication information for schemes requiring a login, in the form of a user name and password.
  
  
• <host>: An Internet host, usually specified either as a fully qualified DNS domain name, or an IP address in dotted-decimal notation.
  
  
• <port>: A TCP or UDP port number to use when invoking the protocol appropriate to the scheme.
  
  
• <url-path>: A resource location path. This is usually a full directory path expressing the sequence of directories to be traversed from the root directory to the place where the resource is located, and then the resource's name. For example, if on a device there is a directory called “project1” and within it a subdirectory called “memos” containing a text file called “June11th-minutes.txt”, the URL path “project1/memos/June11th-minutes.txt” would refer to that resource.
  
  Note that the slash before the <url-path> is required, and while it is technically not considered part of the path, it serves the purpose of “acting” like the slash denoting the root directory in many file systems. Also, the <url-path> may end in a slash, which means that the path refers specifically to a directory. However, this is often not required, as the server will treat the URL as a directory reference by context when needed. A path may also refer to a virtual file, program or resource other than a “normal” file.
  
  
• <params>: Scheme-specific parameters included to control how the scheme is used to access the resource. Each parameter is generally of the form “<parameter>=<value>”, with each parameter specification separated from the next using a semi-colon.
  
  
• <query>: An optional query or other information to be passed to the server when the resource is accessed.
  
  
• <fragment>: Identifies a particular place within a resource that the user of the URL is interested in.

Figure 287 illustrates this common syntax and its elements using an example HTTP URL.