505 Subfield $u - Uniform Resource Identifier:

Subfield $u contains the Uniform Resource Identifier (URI), for example a URL or URN, which provides electronic access data in a standard syntax.  This data can be used for automated access to an electronic item using one of the Internet protocols.

Example:

505  0#  $uhttp://lcweb.loc.gov/catdir/toc/99176484.html

Display example:

Contents: http://lcweb.loc.gov/catdir/toc/99176484.html