In direct delivery, the sender uses the senders IP address to find the destination physical address. This mapping process can be done by Address Resolution Protocol (ARP). If the destination host is not on the same network as the source host, the packet will be delivered indirectly. In an indirect delivery, the packet goes from router to router through a number of networks until it reaches one that is connected to the same physical network as its final destination. Thus, the last delivery is always a direct delivery, which always occurs after zero or more indirect deliveries. In an indirect delivery, the sender uses the destination IP address and a routing table to find the IP address of the next router to which the packet should be delivered. The sender then uses the ARP to find the physical address of the next router. 2.1.2 Address Resolution Protocol (ARP) IP (logical) addresses are assigned independently from physical (hardware) addresses. The logical address

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

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 ma

Figure 2.4 shows an example of simplified ARP dynamic 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 machines 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 1s (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 announcin