ARP – Address Resolution Protocol

ARP or Address Resolution Protocol is used to map Internet addresses to physical addresses. Address resolution is the process of mapping Internet IP addresses into physical addresses. Two machines in the internet cannot communicate with each other  without knowing each others actual physical address i.e address of its hardware interface, for e.g. the network interface card. Address resolution in TCP/IP solves this problem of mapping higher level I{ addresses to physical addresses by special protocol sets. Some of these protocols keep tables in each machine that stores pairs of IP addresses and corresponding physical addresses, while some of them encodes the hardware or the physical address along with the IP address.

According to the type of the network, the physical addresses also vary. For e.g, we can have different physical address schemes for the Ethernet and ProNET 10 for Toke rings, and this different methods of address resolution.

Address resolution by direct mapping for ProNET 10

ProNET 10 for Token ring networks have small physical address or the hardware address of the network interface card. The physical addresses are in such a way that it could be easily extracted from the machines’ IP address i.e if the IP addresses of some ProNET 10 Token ring machines are 98.2.3.1, 98.2.3.2, 98.2.3.3 etc, then hardware or physical addresses of these machines can be 1, 2, 3 respectively i.e. the last byte of the host id part. Such a process of address resolution is efficient and requires lesser computation and no reference to external data or tables. In some cases, we may also use a function f that maps the IP address I to machine’s physical address P as f(I) = P. In cases as in X-25 networks, the physical addresses are stored in tables along with the IP addresses since cannot derive the physical as in the above case. In this case, address resolution software uses hash functions to efficiently restore the physical address corresponding to an IP address.

ARP using Dynamic Binding for Ethernet

Ethernet’s hardware address is 48 bits long and is difficult to be resolved from the IP address directly as in the above case, one reason being that the 48 bits hardware address cannot be included within 32 bit IP address. ARP or address resolution protocol is a lower level protocol that aids in creating an IP address to hardware address binding, especially for the Ethernet type networks employing TCP/IP. ARP is a part of physical network system and not a part of internet protocols.