SNMP Message Format
Figure 1. The OID identifies managed objects that can have assigned values

What terms are defined in the MIB?

The elements defined in the MIB can be extremely broad (for example, all objects created by private businesses) or they can be extremely specific (like a particular Trap message generated by a specific alarm point on an RTU.)

Each element in the MIB is given an object identifier, or OID. An OID is a number that uniquely identifies an element in the SNMP universe. Each OID is associated with a human-readable text label.

What is the function of an OID?

The OIDs identify the data objects that are the subjects of an SNMP message. When your SNMP device sends a Trap or a GetResponse, it transmits a series of OIDs, paired with their current values.

The location of the OID within the overall SNMP packet is shown in Figure 1.

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What does an OID look like?

SNMPMIBTree.jpg

Figure 2. A branch of the MIB object identifier tree

Here's an example: 1.3.6.1.4.1.2681.1.2.102

OK ... but what does it mean?

The OID is a kind of address. It locates this particular element within the entire SNMP universe. The OID describes a tree structure, as shown in Figure 2, and each number separated by a decimal point represents a branch on that tree.

The first few numbers identify the domain of the organization that issued the OID, followed by numbers that identify objects within the domain. Imagine if your home address started "Universe, Milky Way Galaxy ..." and ended with your house number. In a similar way, each OID begins at the root level of the OID domain and gradually becomes more specific.

Each element of the OID also has a human-readable text designation. From left to right, our sample OID reads:

1 (iso): The International Organization for Standardization, one of the two organizations that assign OID domains.
3 (org): An ISO-recognized organization.
6 (dod): U.S. Department of Defense, the agency originally responsible for the Internet.
1 (internet): Internet OID.
4 (private): Private organizations.
1 (enterprises): Business enterprises.
2682 (dpsInc): DPS Telecom.
1 (dpsAlarmControl): DPS alarm and control devices.
2 (dpsRTU): DPS remote telemetry unit.
102 (dpsRTUsumPClr): A Trap generated when all the alarm points on an RTU are clear.