B.13. Structure of a MIB File
When you read a MIB file, you will notice that it is named and defined between BEGIN and END statements. It will describe what it includes from other MIB files, and it will name the module being defined in this MIB file, the contact information for the person responsible for the MIB, and the revision history. Next you will see the definition of the objects in the MIB.
If you look at the IF-MIB, again distributed with the Net-SNMP package, you can see all of these parts. The MIB is enclosed within these lines
IF-MIB DEFINITIONS ::= BEGIN END
The first section states the external dependencies.
IMPORTS MODULE-IDENTITY, OBJECT-TYPE, Counter32, Gauge32, Counter64, Integer32, TimeTicks, mib-2, NOTIFICATION-TYPE FROM SNMPv2-SMI ...