In telecommunications and computer networking, a directory service is a function that maps to and from human-readable names and computer-readable addresses, not intended to be a search engine. By not being a search engine, it requires knowledge of unambiguous or minimally ambiguous computer-friendly identifiers and relatively human-friendly identifiers.
To draw the analogy to the well-known telephone directory service, the user commonly requests the telephone number corresponding to a name. If there are multiple subscribers with the name "Starbucks Coffee", the operator may need to ask disambiguating questions such as location of the particular store.
The "reverse lookup" function, less generally available to telephone users, also has much less ambiguity. In most cases, only one name corresponds to a given telephone number or computer identifier. There are exceptions when calls are automatically forwarded from multiple names to the same number, or where there may be aliases for a number (e.g., local police department name, the number dialed to reach a public safety answering point, such as 911 or 112.
In the Internet, the primary directory service is the Domain Name System. Over time, it has acquired additional applications for security and management, but it is not intended to find things by concept alone. A search engine can look for all restaurants offering coffee in a given location; DNS cannot.
Various proprietary directory services, such as Microsoft Active Directory, can have intermediate capabilities, such "get the nearest printer".