Entry
Network: Address: MAC: What is a MAC address?
Mar 25th, 2005 23:47
Knud van Eeden,
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--- Knud van Eeden --- 20 January 2004 - 10:39 pm --------------------
Network: Address: MAC: What is a MAC address?
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MAC address = 'M'edia 'A'ccess 'C'ontrol 'address'
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In computer networking a media access control address (MAC address) is
a code on most forms of networking equipment that allows for that
device to be uniquely identified.
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For example:
Network interface cards (e.g. a network PCMCIA card which you plug in
in a laptop) are the basic units of communication between computers on
a network.
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Each network interface has to be uniquely identified in the network, so
that it is known where to send the data to.
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This identification of each network card is done by a unique address.
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Network adapter cards use a burned-in address, usually called a Media
Access Control (MAC) address.
This unique hardware address (called MAC address) is burned into a ROM
chip on that network interface card.
Note: this MAC addresses can be changed by you on some hardware
e.g. by using some software
(this process of changing the MAC address is called 'spoofing')
http://en.wikipedia.org/wiki/MAC_address
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This hardware address on the network interface card provides the
definitive address for that computer network card on the network.
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An example of a MAC address is:
00-0B-DB-17-2E-43
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The MAC address assigned to each adapter is unique and is made up of a
6-byte address (48 bits), which is usually expressed in hexadecimal
notation to make it easier to write.
e.g.
00-80-C8-EA-AA-7E is much easier to write than trying to express the
same address in binary, which would be a string of zeros and ones 48
bits long (in this example,
000000001000000011001000111010101010101001110011).
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A MAC address typically looks like:
00-0B-DB-17-2E-43
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This MAC address is a 48 bit number.
(or thus 6 times an 8 bit number, or thus 6 x 8 = 48 bits)
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e.g.
1. here such a 48 bits number:
000000000000000000000000000000000000000000000001
2. if you split this 48 bits in 12 groups of 4 bits:
0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 0001
3. and split this again in 6 groups by dashes between each 8 bits:
0000 0000 - 0000 0000 - 0000 0000 - 0000 0000 - 0000 0000 - 0000 0001
4. and the convert each 4 bits to its hexadecimal representation:
0 0 - 0 0 - 0 0 - 0 0 - 0 0 - 0 1
5. you have its hexadecimal representation of 6 groups of 2
hexadecimal characters:
00-00-00-00-00-01
6. or thus the MAC address
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These MAC addresses are determined by the manufacturer of the network
card.
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On an Ethernet card, for example, a MAC address is composed of two
parts.
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The first 3 bytes of the MAC address identifies the manufacturer of the
network card.
e.g.
00-0B-DB
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Thus the first 3 bytes are fixed, and assigned to a specific
manufacturer (this is done by the IEEE organization).
Thus given the MAC address, you could possibly find out who
manufactured
that device.
To find out the vendor, given the first 3 bytes of the MAC address,
use e.g the URL.
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Vendor/Ethernet MAC Address Lookup and Search
http://www.coffer.com/mac_find/
http://knowledgestorm.techtarget.com/searchcio/search/keyword/MAC+addre
ss/TT/MAC+address
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The remaining 3 bytes are assigned, usually serially, to the
cards that manufacturer produces.
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e.g. for the first 10 cards of that manufacturer the last
3 bytes are as follows:
... 17-2E-00
... 17-2E-01
... 17-2E-02
... 17-2E-03
... 17-2E-04
... 17-2E-05
... 17-2E-06
... 17-2E-07
... 17-2E-08
... 17-2E-09
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As you first send your data between computers using their IP addresses
(e.g. to 192.168.0.1), internally in this computer it has to be
converted
further to the more fundamental hardware MAC address of that network
interface card.
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So there has to be one more conversion round from IP information to MAC
address (similar to the conversion between a URL to an IP address).
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URL -> DNS -> to another computer IP address -> network card (MAC
address)
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e.g.
http://www.mycompany.com -> 192.168.0.1 -> 00-0B-DB-17-2E-43
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This Media Access Control (MAC) layer is here actually a sublayer of
the OSI model's Data Link layer
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This MAC address should be a rather unique number in the network at any
given moment.
Otherwise a network conflict message should be returned, e.g.
by the operating system.
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The MAC address is e.g. used by network switches to determine
the flow of data through its ports.
It creates a table of its port numbers versus the MAC address
of the devices connected to this ports (by first analyzing the
IP frames which flow through each port, and getting the MAC address
from this).
If the switch does not know the MAC address, it will send that package
through all ports (so called 'floading'), to all devices.
If the device sends an IP packet with a MAC address equal to
its own MAC address, the switch will drop that packet.
If the device sends an IP packet with a MAC address not equal to
its own MAC address, it will forward that packet to that MAC address,
or thus to the port to which this MAC address device is connected.
In this way the data is a kind of 'presorted', so making for a more
efficient flow of data.
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Internet: see also:
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MAC address
http://en.wikipedia.org/wiki/MAC_address
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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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