Early Address Resolution Schemes When the Internet first came banging into the public psyche, most users were ultimately connected to the Internet via an Ethernet LAN. LANs use a local device address known as a medium access control (MAC) address, which is 48 bits long and nonhierarchical, which means that it cannot be used in IP networks for routing. To get around this disparity, and to create a technique for relating MAC addresses to IP addresses, the Address Resolution Protocol (ARP) was created. ARP allows a host to determine a receiver's MAC address when it only knows the device's IP address. The process is simple: The host transmits an ARP request packet that contains the MAC broadcast address. The ARP request advertises the destination IP address and asks for the associated MAC address. Since every station on the LAN hears the broadcast, the station that recognizes its own IP address responds with an ARP message that contains its MAC address. As ARP became popular, other address management protocols came into play. Reverse ARP (RARP) gives a diskless workstation (a dumb ter- minal, for all intents and purposes) the ability to determine its own IP address, knowing only its own MAC address. Inverse ARP (InARP) maps are used in frame-relay installations to map IP addresses to frame- relay virtual circuit identifiers. ATMARP and ATMInARP are used in ATM networks to map IP addresses to ATM virtual path/channel iden- tifiers. And finally, LAN Emulation ARP (LEARP) maps a recipient's ATM address to its LAN Emulation (LE) address, which is typically a MAC address. Routing in IP Networks There are three routing protocols that are most commonly used in IP net- works: the Routing Information Protocol (RIP), the Open Shortest Path First (OSPF), and the Border Gateway Protocol (BGP). OSPF and RIP are used primarily for intradomain routing-within a company's dedicated network, for example. They are sometimes referred to as interior gateways protocols. RIP uses hop count as the measure of a particular network path's cost