About MIBs
A Management Information Base (MIB) is a formal description of a set of network objects that can be managed using the Simple Network Management Protocol (SNMP).
The unit of data collected is called the SNMP object. For each device, a set of SNMP objects and rules for addressing the objects are defined in a MIB file. MIBs are key to the logical, orderly functioning of SNMP. MIB objects (OIDs) are represented by a tree hierarchy; each object has a unique address based on its position in the tree. The address count begins from the root of the object tree; one number is added to the address with each new branch. The root of the tree is unnamed and splits into three main branches: Consultative Committee for International Telegraph and Telephone (CCITT), International Organization for Standardization (ISO), and joint ISO/CCITT.
 
Figure 60: The MIB objects (OIDs) tree
 
These branches and those that fall below each category have short text strings and integers to identify them. Text strings describe object names, while integers allow computer software to create compact, encoded representations of the names.
The object identifier in the Internet MIB hierarchy is the sequence of numeric labels on the nodes along a path from the root to the object. The Internet standard MIB is represented by the object identifier 1.3.6.1.2.1. It also can be expressed as iso.org.dod.internet.mgmt.mib. The format of the MIB is defined as part of the SNMP. (All other MIBs are extensions of this basic management information base.) MIB-I refers to the initial MIB definition; MIB-II refers to the current definition. SNMPv2 includes MIB-II and adds some new objects.
Each MIB has a name, a syntax, and an encoding.
Name—identifies the object
1.3.6.1.2.1.1.1=the object identifier
SYSDESCR=the object descriptor
Syntax—defines the object’s structure (e.g., octet string, integer).
Encoding—an object’s representation using the object’s syntax (e.g., the local IP address for this TCP connection, Read Only, or Mandatory).
Object: TCPConnLocalAddress
Syntax: Integer
Definition: The local IP address for this TCP connection
Access: Read only
Status: Mandatory
When requested, the SNMP agent transfers an SNMP message across the network in a standard format, as specified by the set of SNMP Request for Comments (RFCs). Related MIB objects often are combined into MIB groups. MIB groups make it easier to manage a large number of MIB objects. Some MIBs, such as the standard MIB-2, contain many MIB groups. Proprietary MIBs usually have only one, or a few, groups.