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Address Resolution Protocol
(ARP) is the
means by which networked computers map Internet Protocol
(IP)
addresses to physical hardware (MAC) addresses that are recognized in
a local network. ARP builds and maintains a table called the ARP
cache, which contains these mappings (IP address-MAC address). The ARP
cache is the means by which a correlation is maintained 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.
There are several switches that can be
used with the ARP
command:
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arp -a – Displays the cache
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arp -s – Adds a permanent IP-to-MAC
mapping
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arp -d – Deletes an entry from the
ARP cache
There are other switches included with
specific vendor implementations of ARP.
RARP
Machines that do not know their IP addresses use Reverse Address Resolution
Protocol (RARP). It is used to obtain IP address
information based on the physical or MAC address. RARP provides the
rules by which the physical machine in a LAN can request to learn its
IP address from a gateway server Address Resolution Protocol
(ARP)
table or cache. A gateway server is a computer or router that is
configured to receive information from computers in the local network
and send it to computers in a remote location such as the Internet or
other areas in a large internetwork.
Note: ARP maps IP-to-MAC addresses. RARP is the reverse
of ARP. It maps MAC-to-IP addresses
NSLOOKUP.EXE
Another utility, nslookup, returns the IP address for a given
hostname. It will also do the reverse and find the host name for a
specified IP address. For example, entering "cisco.com", would deliver
198.133.219.25, which is the Cisco IP address. Entering
"198.133.219.25", would deliver "cisco.com".
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