The ANSI information retrieval scheme protocol (Z39.50) is used for simple plain language text-based database queries. This pre-Internet protocol was originally planned as an electronic card file system for physical libraries. Server access requires using a Z39.50 client to connect to a database known as a "Target". To access a Target, the Client needs to know the URL with Port number, and the Target name (<server>:<port>/<target path>). The URI that precedes the address is "tcp:". Annoyingly, many Targets held on different Servers have identical (default) names, due to the fact that many of the databases were originally created as in-house stand-alone systems (sometimes several in large libraries with multiple collections) by the same commercial library automation company. Be aware that a Target database can be constructed using any of several formatting schemes, called a "Record Type". The four most popular Record Types are marc21, opac, sutrs, and xml. Many Targets are now accessible through the Internet, so below are addresses to various free public access Z39.50 servers.
[IANA Port assignment = 210]

Z39.50 Targets