assets should be active because the SNMP agent running on the assets must reply to queries. Network administrators must update sysLocation whenever the asset changes position. This update process is not automated: it relies on human operators and so it is possible that sysLocation has outdated values. Hence this approach does not consti- tute a solution to the asset localisation problem. Asset location generally can only be performed if some support for the task is provided by the underlying network. For example, consider a LAN based on bus Ethernet (10-100Base/2) or token ring. In that case, it is not possible, from a pro- gramming viewpoint, to discover whether two hosts are physically adjacent. On the other hand, point-to-point networks such as ATM or 10Base/T Ethernet do allow localisation of hosts. Once the physical location of network connectors is known 10 Network Inventory Management Gateway/router Subnet A Subnet B Figure 10.4 Subnets attached to a gateway/router. and stored in a database, it is possible to calculate which host is attached to a given connector. This, in turn, localises the host. If the underlying network is point-to- point, hosts can be located independently of the network protocol used by the hosts. JLocator currently works in the IP world only, as IP is the most widely used pro- tocol. The algorithm used to locate hosts relies on the media access control (MAC) address. The MAC address is the hardware address of a device attached to a net- work. Although it differs for various physical media, network board manufacturers guarantee its uniqueness and this allows a computer to be uniquely identified inde- pendently of its IP address. Some hosts implement single link multi-homing, which allows multiple IP addresses to be associated with the same hardware interface. In the Internet world, the Address Resolution Protocol (ARP) is used to map dynamically and correlate MAC and IP addresses. The IETF's RFC 1157 defines the object type atTable that allows, through SNMP, cached ARP tables to be re