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3.2.2. Server-based Naming Authority

URL schemes that involve the direct use of an IP-based protocol to a specified server on the Internet use a common syntax for the server component of the URI's scheme-specific data:

<userinfo>@<host>:<port>

where <userinfo> may consist of a user name and, optionally, scheme specific information about how to gain authorization to access the server. The parts "<userinfo>@" and ":<port>" may be omitted.

server = [ [ userinfo "@" ] hostport ]

The user information, if present, is followed by a commercial at-sign "@".

userinfo = *( unreserved | escaped |

";" | ":" | "&" | "=" | "+" | "$" | "," )

Some URL schemes use the format "user:password" in the userinfo field. This practice is NOT RECOMMENDED, because the passing of authentication information in clear text (such as URI) has proven to be a security risk in almost every case where it has been used.

以上、urn:ietf:rfc:2396 より