STD - Subscriber Trunk Dailing
STD or 'Subscriber Trunk Dialing' codes are designed to dial long distance calls with out any operator assistance. In India, there are 2646 STD Codes under 22 Circles. Each circle is divided into telecom districts under which local telephone exchanges assign numbers to the individuals. Subscriber Trunk Dialing or STD system applies only to land-line numbers. There are public and private networks who operate land-line numbers in India.
To trace any land-line/STD number and to know the network operator, location details, you can use this Number Tracker. To search more STD codes using fuzzy logic, you can use the STD Code Search tool.