SNMP is based on the manager/agent model consisting of a manager, an agent, a database of management information, managed objects and the network protocol. The manager provides the interface between the human network manager and the management system. The agent provides the interface between the manager and the physical device(s) being managed, such as bridges, hubs, routers or network servers, these managed objects might be hardware, configuration parameters, performance statistics, and so on.
The MIB is organized in a tree structure with individual variables, such as point status or description, being represented as leaves on the branches. A long numeric tag or object identifier (OID) is used to distinguish each variable uniquely in the MIB and in SNMP messages. The MIB lists the unique object identifier of each managed element in an SNMP network. The SNMP manager canít monitor devices unless it has compiled their MIB files. The MIB is also a guide to the capabilities of SNMP devices. For example, if MIB lists OIDs for Traps but not for GetResponse messages, it will report alarms, but will not respond to alarm polls.
Each SNMP element manages specific objects with each object having specific characteristics. Each object / characteristic has a unique object identifier consisting of numbers separated by decimal points (i.e., 126.96.36.199.4.1.2682.1). These object identifiers naturally form a tree as shown below. The MIB associates each OID with a readable label and various other parameters related to the object.
Here's a list of MIB files offered by Cisco: