SNMP MIB objects, groups, and addresses
A MIB is a set of SNMP objects organized in a tree address structure. Each object in a MIB has a unique address called an object identifier, and each branch on the tree is identified by a number.
 
The ISO 8824 specification defines the lower branches of the SNMP MIB tree as:
iso(1).org(3).dod(6).internet(1) or, as expressed in Structure of Management Information (SMI) language, 1.3.6.1 (see illustration).
The SNMP tree resides under the Internet subtree. Four branches after the Internet subtree can be used by SNMP: The directory(1) subtree is reserved for future use by OSI.
The mgmt(2) subtree includes standard SNMP MIBs I or II (RFC1156 and RFC1213).
The experimental(3) subtree is reserved for Internet experiments.
The private(4) subtree provides space for vendor-specific MIBs. All private MIBs are located under enterprises(1) branch. Any private object ID (OID) should begin from the base address 1.3.6.1.4.1.
The address 1.3.6.1.2.1 or iso.org.dod.internet.mgmt.mib represents the address of the standard SNMP MIB I or II on the ISO tree. Inside the MIB branch, SNMP objects are organized beneath higher level branches called MIB groups. Because of the large number of objects—the standard MIB II includes almost two hundred—MIB groups have been created to simplify addressing. Groups consist of related objects: for example, ICMP, TCP, EGP, and other statistics object groups.
The object address is the path from the MIB's root to an object. For example, the object sysDescr in the MIB(1) System Group has the address 1.3.6.1.2.1.1.1 (see illustration).