Cisco Networking: MAC Addressing
The MAC address is taken by default from the hardware address of the network card. The MAC address is a 12-digit hexadecimal number or 48 bits in length. This address is assigned by the hardware manufacturer and is globally unique, so you should not have duplicate addresses on your network (although cards with duplicate addresses have been manufactured by production errors in the past).
Typically, the problem of duplicate MAC addresses arises because people choose not to use the vendor-assigned hardware address, but instead use a self-assigned address (also called a locally administered address). This is a technique used by hackers to circumvent MAC-based security restrictions. This is more common when using main frame systems that communicate via MAC addresses rather than protocol addresses (such as IP addresses).
In the later case, if a computer or its network card is replaced due to a hardware failure, you would have to reconfigure several systems to work with the new MAC address, so it is far easier to assign the new network card the same MAC address as the failed card. Unless you are in the small minority of people with a system such as this, or are a big time hacker, you can safely ignore the ability to manage your own MAC address.
IEEE operates an online database where you can look up MAC addresses at IEEE Standards The first half of the twelve digit hexadecimal MAC address is used to register hardware vendors, and the second half of the address is assigned by the hardware vendor. The following table shows a list of the first 20 companies that have addresses registered.
|Xerox Corporation, USA||00-00-00 – 00-00-09|
|Omron Tateisi Electronics Co., Japan||00-00-0A|
|Matrix Corporation, USA||00-00-0B|
|Cisco Systems, Inc., USA||00-00-0C|
|Fibronics LTD., Israel||00-00-0D|
|Fujitsu Limited, Japan||00-00-0E|
|Next, Inc., USA||00-00-0F|
|Sytek Inc., USA||00-00-10|
|Normerel Systemes, France||00-00-11|
|Information Technology Limited, United Kingdom||00-00-12|
|Datapoint Corporation, USA||00-00-15|
|Du Pont Pixel Systems, United Kingdom||00-00-16|
|Webster Computer Corporation, USA||00-00-18|
|Applied Dynamics International, USA||00-00-19|
|Advanced Micro Devices, USA||00-00-1A|
|Novell Inc., USA||00-00-1B|
|Bell Technologies, USA||00-00-1C|
The companies that were first to register IDs on this list were some of the first companies in the networking field. Reviewing even the first few entries on the table, you notice some of those names as incumbent or early starters in the networking and computing world, including Xerox (which actually has the first ten registered numbers), Cisco, Fujitsu, and so on.
By using the company registration as half of the MAC address length, there can be almost 17 million registered network card manufacturers, and each registered company can manufacture almost 17 million network cards.
If you are unlucky enough to get duplicated MAC addresses on your network, either by hardware vendor error or by using locally assigned MAC addresses, you are in for some network troubleshooting joy.
A duplicated MAC address means you have multiple devices responding to data requests as if they are the only device with that address on the network, or you have a switch that keeps changing the port assignment for that address because the switch keeps seeing the device’s MAC address moving from port to port.
This causes any number of issues with network connectivity, as you can probably guess. Hopefully, knowing that duplicate MAC addresses might happen, you may not dismiss it as an impossibility, which will allow you to identify the problem more quickly.