Each URL scheme has its own rules regarding the presence or absence of the syntactic components described in Sections 2.1 and 2.2. In addition, some schemes are never appropriate for use with relative URLs. However, since relative URLs will only be used within contexts in which they are useful, these scheme-specific differences can be ignored by the resolution process.
Within this section, we include as examples only those schemes that have a defined URL syntax in RFC 1738 . The following schemes are never used with relative URLs:
mailto Electronic Mail news USENET news telnet TELNET Protocol for Interactive Sessions
Some URL schemes allow the use of reserved characters for purposes outside the generic-RL syntax given above. However, such use is rare. Relative URLs can be used with these schemes whenever the applicable base URL follows the generic-RL syntax.
gopher Gopher and Gopher+ Protocols prospero Prospero Directory Service wais Wide Area Information Servers Protocol
Users of gopher URLs should note that gopher-type information is almost always included at the beginning of what would be the generic-RL path. If present, this type information prevents relative-path references to documents with differing gopher-types.
Finally, the following schemes can always be parsed using the generic-RL syntax. This does not necessarily imply that relative URLs will be useful with these schemes -- that decision is left to the system implementation and the author of the base document.
file Host-specific Files ftp File Transfer Protocol http Hypertext Transfer Protocol nntp USENET news using NNTP access
We recommend that new schemes be designed to be parsable via the generic-RL syntax if they are intended to be used with relative URLs. A description of the allowed relative forms should be included when a new scheme is registered, as per Section 4 of RFC 1738 .