Entry
Network: Internet: Protocol: ARP: What is ARP?
Mar 25th, 2005 23:52
Knud van Eeden,
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--- Knud van Eeden --- 20 January 2004 - 09:59 pm --------------------
Network: Internet: Protocol: ARP: What is ARP?
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ARP = Address Resolution protocol.
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This is a protocol that maps IP addresses (=network based)
to MAC (=hardware based) addresses.
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The ARP protocol is a low-level TCP/IP protocol used to obtain the
physical address of a device when only its logical IP address is known.
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When communicating on the same network segment, a computer can
communicate directly by sending directed datagrams to another computer
and specifying the unique MAC address that is burned into the network
card when it is manufactured.
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In TCP/IP networks, the Address Resolution Protocol (ARP) is used in a
local broadcast domain to determine the hardware address of another
computer by sending out a broadcast packet that contains the computer's
IP address. When a computer recognizes an ARP packet that has its IP
address, it responds to the ARP request with another packet that tells
the original computer what the destination computer's MAC address is.
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e.g.
Given your IP address (e.g. 123.012.125.10)
what is your MAC address? (e.g. 00-01-02-AB-CD-EF)
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The steps are as follows:
1. An ARP request with the IP address is broadcast onto the network,
by the device that needs to know this:
(that is something like the below conversation
is sent to all devices on that network:
'which device has this IP address?
If it is you, can you send your MAC address back to my IP
address?'
2. The device with that IP address sends back its hardware MAC address
so that packets can be transmitted.
(that is something like:
'I am the device with that IP address,
and I send you now my MAC address back (to your IP address)'
3. All this entries are stored in a table of IP address versus MAC
address, on that device
------------------------------------------------------------------
DATABASE: TABLE: TYPE: ONE TO ONE
------------------------------------------------------------------
| NR | IP ADDRESS | MAC ADDRESS |
------------------------------------------------------------------
1 123.012.125.10 00-01-02-AB-CD-EF
2 192.168.193.152 0B-0C-0D-00-11-00
... ... ...
------------------------------------------------------------------
4. So next time the device can immediately get the MAC address
information given the IP address from this table
(so without having to wait for the result of a broadcast sent out,
which might take a longer time)
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You can e.g. yourself checking this communication between the devices
on the network, by using a sniffer like Ethereal.
http://www.ethereal.com/download.html
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To find out more about this conversion from IP address to MAC address
in Microsoft Windows, and the IP address versus MAC address table,
use e.g the program 'arp.exe'.
Type e.g. on the MSDOS command line the command:
arp.exe -a
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Internet: see also:
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Network: Links: Overview: Can you give me an overview of links?
http://www.faqts.com/knowledge_base/view.phtml/aid/34809/fid/625
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