MAC address issue

Hi I am getting confused about the issue. Anyone can give me suggestion? MAC is physical address that is assigned by factory. It can not be changed by some command. Why do we have the command "mac-address aaaa.bbbb.cccc" under interface mode , which can change mac address ? Thank you
EESkyAsked:
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Don JohnstonConnect With a Mentor InstructorCommented:
No. MAC addresses are 48-bits long (6-bytes) which are expressed in hexadecimal. Period.

Whether you choose to put a hyphen between each byte (xx-xx-xx-xx-xx-xx) or a period between every two bytes (xxxx.xxxx.xxxx) is up to you or the manufacture. In your example, the a's, b's and c's are just letters. Just like you could have used all x's or x, y and z.  The a's don't represent anything other than this is where the first two bytes of the MAC address are. The b's would be the third and fouth bytes.
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Dave BaldwinConnect With a Mentor Fixer of ProblemsCommented:
While originally MAC addresses were fixed at the factory, that is no longer true.  The MAC address on many devices like NICs and routers can be changed.  I don't know when they started doing that but a big incentive was when MAC addresses needed to be 'cloned' by routers to enable them to work with DSL and other services that were 'tied' to a single MAC address.  I think the ISPs have pretty much given up on that restriction since the routers get around it so easily now.
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msifoxConnect With a Mentor Commented:
Even back in times of DOS programs using packet drivers, it was the software that copied the MAC into the packet to be sent. It could have copied any MAC there. But usually it would use the one that was stored in the NIC, because then it's guaranteed to be unique. The MAC stored in the hardware is an offer that the software can use, but it doesn't need to.
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Don JohnstonConnect With a Mentor InstructorCommented:
MAC addresses are still set at the factory. They are burned in to the ROM on the interface. However, during initialization of the interface, the MAC address is copied from ROM to an area of interface RAM called the address buffer. Because the MAC address is compared to the destination address of every frame received, a performance improvement is realized by checking against the address in RAM as opposed to ROM.

Since the working address is in RAM, it's possible to change it.  I haven't run into a situation in a LONG time where that was necessary though. For example, back in the days of AS400's and PC's running terminal emulation, the MAC address was one of the ways to control access. If a network card was replaced, many times it was simply easier to change the MAC of the new NIC to that of the old NIC rather than request a change in the access rules on the AS400.

In the case of cable modems, some service providers lock their equipment to the first MAC address that's seem coming from your house. If that happens to be your PC, if you then try to install a router, the provider won't allow the traffic since it's coming from a different MAC address. You could call them and ask that the clear that entry. Or just change the MAC of your router to the MAC of your PC.
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EESkyAuthor Commented:
Thank all of you for the explanation. Usually mac address is expressed in the format: xx-xx-xx-xx-xx-xx, which is hexadecimal format. but in router or switch we use command: "mac-address aaaa.bbbb.cccc" What portion does the aaaa.bbbb.cccc represent in xx-xx-xx-xx-xx-xx ?
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Don JohnstonConnect With a Mentor InstructorCommented:
Sadly, there is no standard for expressing MAC addresses. :-(

The only portions of the MAC address that are significant is the first 3 bytes. Those are the OUI (also know as the vendor ID) which correspond to the manufacture of the interface. For example, Cisco's first OUI was 00-00-0C. So every ethernet interface on the early Cisco routers had MAC addresses starting with 00-00-0C. The remaining 3 bytes are serialized by the manufacture. So the very first interface on the very first Cisco router probably had a MAC address of 00-00-0C-00-00-01. And the second interface would have had a MAC of 00-00-0C-00-00-02. And so on.
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EESkyAuthor Commented:
In command "mac-address aaaa.bbbb.cccc", aaaa.bbbb.cccc represents last three octet(bytes) that you mentioned, is it what you meant ?
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EESkyAuthor Commented:
So, the "aaaa.bbbb.cccc" in that command represents whole 48 bits mac address, right ?
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Don JohnstonConnect With a Mentor InstructorCommented:
Yes. In hex, each character represents 4-bits.
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EESkyAuthor Commented:
Excellent, Thank you !
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