What is ARP? What is ARP Cache Poisoning? Basic Netwroking

What is ARP? What is ARP Cache Poisoning?
Address Resolution Protocol (ARP) is a protocol for mapping an Internet Protocol address (IP
address) to a physical machine address that is recognized in the local network. For example,
in IP Version 4, the most common level of IP in use today, an address is 32 bits long. In an
Ethernet local area network, however, addresses for attached devices are 48 bits long. (The
physical machine address is also known as a Media Access Control or MAC address.) A
table, usually called the ARP cache, is used to maintain a correlation between each MAC
address and its corresponding IP address. ARP provides the protocol rules for making this
correlation and providing address conversion in both directions.
How ARP Works

When an incoming packet destined for a host machine on a particular local area network
arrives at a gateway, the gateway asks the ARP program to find a physical host or MAC
address that matches the IP address. The ARP program looks in the ARP cache and, if it finds
the address, provides it so that the packet can be converted to the right packet length and
format and sent to the machine. If no entry is found for the IP address, ARP broadcasts a
request packet in a special format to all the machines on the LAN to see if one machine
knows that it has that IP address associated with it. A machine that recognizes the IP address
as its own returns a reply so indicating. ARP updates the ARP cache for future reference and
then sends the packet to the MAC address that replied.

Since protocol details differ for each type of local area network, there are separate ARP
Requests for Comments (RFC) for Ethernet, ATM, Fiber Distributed-Data Interface,
HIPPI, and other protocols.

There is a Reverse ARP (RARP) for host machines that don't know their IP address. RARP
enables them to request their IP address from the gateway's ARP cache.
RARP (Reverse Address Resolution Protocol) is a protocol by which a physical machine in a
local area network can request to learn its IP address from a gateway server's Address
Resolution Protocol (ARP) table or cache. A network administrator creates a table in a local
area network's gateway router that maps the physical machine (or Media Access Control -
MAC address) addresses to corresponding Internet Protocol addresses. When a new machine
is set up, its RARP client program requests from the RARP server on the router to be sent its
IP address. Assuming that an entry has been set up in the router table, the RARP server will
return the IP address to the machine which can store it for future use.