IP (logical) addresses are assigned independently from physical (hardware) addresses. The logical address is called a 32-bit IP address, and the physical address is a 48-bit MAC address in Ethernet and token ring protocols. The delivery of a packet to a host or a router requires two levels of addressing, such as logical (IP) address and physical (MAC) addresses. When a host or a router has an IP datagram forwarding to another host or router, it must know the logical IP address of the receiver. Since the IP datagram is encapsulated in a form to be passed through the physical network (such as a LAN), the sender needs the physical MAC address of the receiver.
Mapping of an IP address to a physical address can be done by either static or dynamic mapping. Static mapping means creating a table that associates an IP address with a physical address. But static mapping has some limitations because table lookups are inefficient. As a consequence, static mapping creates a huge overhead on the network. Dynamic mapping can employ a protocol to find the other. Two protocols (ARP and RARP) have been designed to perform dynamic mapping. When a host needs to find the physical address of another host or router on its network, it sends an ARP query packet. The intended recipient recognises its IP address and sends back an ARP response which contains the recipient IP and physical addresses. An ARP request is broadcast to all devices on the network, while an ARP reply is unicast to the host requesting the mapping.
mapping. Let a host or router call a machine. A machine uses ARP to find the physical address of another machine by broadcasting an ARP request. The request contains the IP address of the machine for which a physical address is needed. All machines (M1, M2, M3, . . .) on the network receive an ARP request. If the request matches a M2 machine’s IP address, the machine responds by sending a reply that contains the requested physical address. Note that Ethernet uses the 48-bit address of all 1’s (FFFFFFFFFFFF) as the broadcast address.
A proxy ARP is an ARP that acts on behalf of a set of hosts. Proxy ARP can be used to create a subnetting effect. In proxy ARP, a router represents a set of hosts. When an ARP request seeks the physical address of any host in this set, the router sends its own physical address. This creates a subnetting effect. Whenever looking for the IP address of one of these hosts, the router sends an ARP reply announcing its own physical address. To make address resolution easy, choose both IP and physical addresses the same length. Address resolution is difficult for Ethernet-like networks because the physical address of the Ethernet interface is 48 bits long and the high-level IP address is 32 bits long. In order for the 48-bit physical address to encode a 32-bit IP address, the next generation of IP is being designed to allow 48-bit physical (hardware) addresses P to be encoded in IP addresses I by the functional relationship of P = f (I).
