 example, when the MIB module is loaded in an MIB browser, the label of the variable, such as sysDescr, can be used to identify it, since the MIB browser can use the MIB module to translate this label to an OID by querying the MIB database. A key aspect of MIB is that only the types of object on the managed device are spec- ified by the MIB, and not the specific objects/instances. For example, ifInOctets in RFC1213-MIB6 specifies a type of object for a number of input octets on an interface. However, the specific objects or instances of that type are specified as ifInOctets.1, ifInOctets.2, , ifInOctets.n, where n is number of interfaces to the managed device. SNMP (version 1) supports five different kinds of message: GetRequest, SetRequest, GetNextRequest, GetResponse and Trap. A single SNMP message is a PDU, whose format is in accordance with the ASN.1 notation. The PDUs are translated into binary format using Basic Encoding Rules (BER). Binary format is the most basic form in which commands can be expressed and is the form in which they are directly understood by the managed device. SNMP request messages are sent from managers to agents. Request messages can poll the agent for current performance or configuration data, ask for the next SNMP object in an MIB or modify configura- tion settings. Polling is where a request for information is sent electronically at regu- lar intervals, such as every second or every minute. The purpose of an SNMP agent is to decode request messages reliably and process the resulting application data. To specify an object to an SNMP agent, both the Object Name or ID (OID), which defines the type of object, and the instance, the specific object of the given type, 6 Request for Comment number 1213, published by the IETF. 4 Network Management Using SNMP SNMP management application PC running network management software UDP packet PDU in ASN.1 managed object device Figure 4.2 SNMP management application communicating with a managed object. must be provided. An OID is obtained from the MI