Editing Management information base (section)

==MIB hierarchy==
The MIB hierarchy can be depicted as a tree with a nameless root, the levels of which are assigned by different organizations. The top-level MIB OIDs belong to different standards organizations, while lower-level object IDs are allocated by associated organizations. This model permits management across all [[Abstraction layer|layer]]s of the [[OSI reference model]], extending into [[application software|applications]] such as [[database]]s, [[email]], and the [[Java reference model]], as MIBs can be defined for all such area-specific information and operations.

A managed object (sometimes called a MIB object or object) is one of any number of specific characteristics of a managed device. Managed objects are made up of one or more object instances, which are essentially variables. An OID uniquely identifies a managed object in the MIB hierarchy.

Two types of managed objects exist: 
*Scalar objects define a single object instance.
*Tabular objects define multiple related object instances that are grouped in MIB tables.

An example of a managed object is ''<code>atInput</code>'', which is a scalar object that contains a single object instance, the [[integer]] value that indicates the total number of input [[AppleTalk]] packets on a router [[Network interface controller|interface]].

===SNMPv1 and SMI-specific data types===
The first version of the [[Structure of Management Information]] (SMIv1) specifies the use of a number of SMI-specific data types, which are divided into two categories: simple data types and application-wide data types.

====Simple data types====
Three simple data types are defined in the SNMPv1 SMI: 
* The ''integer'' data type is a signed integer in the range of −2<sup>31</sup> to 2<sup>31</sup>−1.
* ''Octet strings'' are ordered sequences of 0 to 65,535 octets.
* ''Object IDs'' represent object identifiers that are allocated according to the rules specified in ASN.1.

====Application-wide data types====
The following application-wide data types exist in the SNMPv1 SMI:
*''Network addresses'' represent addresses from a particular protocol family. SMIv1 supports only 32-bit (IPv4) addresses. SMIv2 uses Octet Strings to represent addresses generically, and thus are usable in SMIv1 too. SMIv1 had an explicit IPv4 address datatype.
*''Counters'' are non-negative integers that increase until they reach a maximum value and then roll over to zero. SNMPv1 specifies a counter size of 32 bits.
*''Gauges'' are non-negative integers that can increase or decrease between specified minimum and maximum values. Whenever the system property represented by the gauge is outside of that range, the value of the gauge itself will vary no further than the respective maximum or minimum, as specified in {{IETF RFC|2578}}.
*''Time ticks'' represent time since some event, measured in hundredths of a second.
*''Opaques'' represent an arbitrary encoding that is used to pass arbitrary information strings that do not conform to the strict data typing used by the SMI.
*''Integers'' represent signed integer-valued information. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI.
*''Unsigned integers'' represent unsigned integer-valued information, which is useful when values are always non-negative. This data type redefines the integer data type, which has arbitrary precision in ASN.1 but bounded precision in the SMI.

===SNMPv1 MIB tables===
The SNMPv1 SMI defines highly structured tables that are used to group the instances of a tabular object (that is, an object that contains multiple variables). Tables are composed of zero or more rows, which are indexed in a way that allows an SNMP manager to retrieve or alter an entire row with a single <code>Get</code>, <code>GetNext</code>, or <code>Set</code> command.

===SMIv2===
The second version of the SMI (SMIv2) is described in {{IETF RFC|2578}} and {{IETF RFC|2579}}. It enhances and adds to the SMIv1-specific data types, such as including bit strings, network addresses, and counters. Bit strings are defined only in SMIv2 and comprise zero or more named bits that specify a value. Network addresses represent an address from a particular protocol family.  Counters are non-negative integers that increase until they reach a maximum value and then return to zero. In SMIv1, a 32-bit counter size is specified. In SMIv2, 32-bit and 64-bit counters are defined.

SMIv2 also specifies information modules, which specify a group of related definitions. Three types of SMI information modules exist: MIB modules, compliance statements, and capability statements. 
* MIB modules contain definitions of interrelated managed objects.
* Compliance statements provide a systematic way to describe a group of managed objects that must be implemented for conformance to a standard.
* Capability statements are used to indicate the precise level of support that an agent claims with respect to a MIB group. An NMS can adjust its behavior toward agents according to the capabilities statements associated with each agent.