Address Resolution Protocol

Address Resolution Protocol:
Every host system, private and public, as well as the routers
connecting their networks each have a globally unique physical
address. The details of this address will be discussed in the next
section. When an IP module requests a datagram be transmitted
by the network layer, be it in an end system or an intermediate
router, IP must first translate between the IP address and this
unique physical address using another Internet layer service, the
Address Resolution Protocol (ARP).

Address Resolution Protocol:
The Address Resolution Protocol is a method for translating
between Internet layer and network layer address. The ARP
module in a computer or router maintains a translation table of
logical to physical mappings it is aware of, called the ARP cache.
If there is not a translation for the address in the table, it will make an ARP broadcast.

Using the Internet and TCP/IP in an example, the basic steps of
ARP translation are:

1. ARP checks the local cache to see if it knows the mapping
between the IP address and a physical address (the
Media Access Control [MAC] address).
2. If there is not a match, ARP broadcasts an ARP request to
the local network. The broadcast is received by every computer and router to which the host is connected.
3. If a computer with a matching IP address exists on the local network, it sends its MAC address back. The ARP
module adds this translation to the ARP cache for future use.
4. If a router on the local network realizes the requested IP address is outside the local network, it sends back its
MAC address, in order for the IP datagram to be forwarded to it.

The same process is used to forward IP datagrams between
routers as a packet transverses the network. When a router is designated as the next hop, the MAC address of the router,
rather than the receiving computer, is provided as the IP to MAC translation.

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