B.5. The OID
Every object in every MIB has an object identifier (OID). An OID is a unique name consisting of a sequence of decimal digits separated by periods, or dots, like this:
This name represents the object's location in the namespace. The first dot represents the root of the tree, and each number represents a node (the base of a branch) in the tree. Everything nameable in SNMP is located somewhere in this OID tree, and every name (OID) contains the entire path to that object from the root of the tree.
This may look strange at first, but it works in the same way as the Unix filesystem, with which you should be familiar. Paths to files start at root, indicated by an initial slash (/), proceed through a number of named nodes ...