Each data class within a Management Information Base (MIB) is defined by an Object Identifier (OID). OIDs are unique across all MIBs. An OID consists of a series of digits separated by periods. The OID functions in a similar fashion to a phone number. The phone number 011-512-471-0000 uniquely identifies a single phone. The phone number can be broken down into a number of components to uniquely identify a phone. The first component, 011, is the country code for the United States. The second component, 512, identifies the area code for central Texas. The third component, 471, is the phone exchange for a large state university in the city of Austin. The final component, 0000, is the main switchboard.
There are two main differences between the phone number example and an actual OID. The first difference is that there are many more components in an OID, up to 128. The combination of these components is called an OID prefix. The second difference is that OIDs support the concept of indexes or keys. The OID prefix specifies the data class but does not specify an instance of the data within the class. Indexes can be used to identify the instances of a data class. These indexes are referred to as the OID suffix.
The assignment of values for each OID prefix component can be illustrated by using a tree structure. The following is an example of an OID assignment:
In the preceding example, the OID prefix for the Dell enterprise would be 184.108.40.206.4.1.674.
The numbers in boldface type show the categories and numbers that apply to Server Administrator. All Server Administrator-defined OIDs consist of 220.127.116.11.4.1.674 followed by additional component values.