Today I found out another interesting bit of information about the MAC address, actually its not just 1 bit its 2 bits.

The most significant byte is the first byte in the address and in that byte the last bit is the most significant bit.

Also the most significant bit is called the Individual/Group (I/G) bit and the next most significant bit is the Universal/Local (U/L) bit.

If the I/G bit is set to 0 it defines that the address is unicast and conversely if it is set to 1 it is multicast or broadcast.

If the U/L bit is set to 0 it means the address is vendor assigned but it if it set to 1 it is administratively assigned, its noted tha many devices do not enforce the rule of the U/L bit.

Automatic Medium-Dependant Interface Crossover is a licenced technology from HP. According to their website at http://www.hp.com/hpinfo/abouthp/iplicensing/automdix.html 200million ports supporting the technology will ship this year.

Its not supported on all Cisco switches, & it prevents the wrong cable from being installed, eg a striaight through between two switches, by simply swapping the transmit and receive pairs if the wrong cable has been used.