The MIB, short for Management Information Base, is the specification that describes the type of data that a particular SNMP agent can provide. Each type of data is defined in the MIB by a unique number, a so called OID (Object Identifier). The unique object identifiers are defined by the position of the object in the tree-structured MIB. However, as such numbers are difficult to remember and thus impractical to use in daily network management, the MIB also contains a corresponding name for each OID to identify them by.

Somewhat confusingly, the term MIB is also sometimes used to designate the actual data which SNMP agents store about themselves and return to the SNMP manager.