Address Resolution Protocol ARP Definition
The hosts or routers are recognized at network layer by IP addresses or by addresses defined by some other protocol used instead of IP. An IP address is an inter network address. However to identify a host irrespective of network protocol, we need an other address called MAC address. These addresses are hard-coded into devices while manufacturing like Ethernet card code.
Address Resolution protocol ARP maps these IP addresses and MAC addresses; which IP address belongs to which MAC address. This mapping can be done statistically, like writing manually into a configuration file, or dynamically by ARP. But static mapping is not useful always because machines may change network cards or machines may go for other physical technology. In such case static mapping task needs to be updated frequently.
Dynamic mapping is done by ARP and RARP (Reverse A.R.P) protocols, A.R.P maps IP addresses to MAC addresses and RARP does the reverse. Suppose a host ‘A’ wants to know MAC address of host ‘B’ for which IP address of ‘B’ is known to ‘A’. So ‘A’ sends ARP packet containing MAC address, IP address of A and IP address of B. This packet will be broadcasted in the network. We can send packets to a host if and only if we know its MAC address directly or indirectly.
All the hosts in the network take that packet. They compare IP address in the packet with their IP address. All except ‘B’ will discard the packet. Only ‘b’ identifies the IP address and fills its MAC address and forms AR Protocol reply packets. It sends it directly to ‘A’. Since ‘B’ can know MAC address of ‘A’ from address resolution protocol request packet. Note that address resolution protocol request is always broadcast in nature (one-to-all) whereas ARP reply is unicast (one-to-one) in nature.