CHAPTER 24 Simple Network Management Protocol 623 To cover all of these vendor-specific situations, the SMI conceptual tree includes a branch for private MIB extensions.The SMI path to the private MIB is 1.3.6.1.4.1 .This leads to the enterprise branch of the SMI tree, where each vendor may obtain a branch number (identifier) and label (descriptor) from the Internet Assigned Number Author- ity (IANA) for the vendor's private MIB. For example, all IBM private MIB objects reside at 1.3.6.1.4.1.2... on the SMI tree because "2" is IBM's enterprise number. Cisco routers use 1.3.6.1.4.1.9... , Hewlett-Packard has 1.3.6.1.4.1.11... , and so forth. More than 700 enterprise code numbers have been assigned by the IANA, showing the wide availability of SNMP-compliant products. This system of private MIBs makes sense because only the manufacturer of the net- work device could possibly know whether the device even has a cooling fan, battery backup, or other hardware feature. Obviously, a network manager would like to know if a device's fan has failed, especially if the device is in a closet where it may overheat and fail after a few hours. The private MIB offers a way of allowing this information to be accessed by the network manager. SNMP manager software will generally have no concept of just where the private MIB objects are and what these objects represent. Some vendors would actually "hide" their private MIB descriptions by limiting their availability, and just what the number 2 in