Subject: Info: MAC Numbers: All you wanted to know, but were afraid to ask. From: Mac_Numbers@a Sender: Mac_Numbers@b DRAFT By JHS You mentioned not knowing what MAC numbers are: First: A MAC number is not an Ethernet IP number. - An Ethernet IP number is usually a 4 byte number of form: [0-255].[0-255].[0-255].[0-255] it is not unique to a PC, there are not enough to go round in the world, so they get re-issued by internet providers on new connections. So they are not used for software licencing. (A bit like in Germany, where if you move to a house in a new town, you have to change your car registered number plates, but the chassis number remains unchanged). 0 to 255 is 256 possibilities. Comes from the 8 bits in a byte, all 8 bits can be in any of 2 states: 0 or 1, so 2 x 2 x 2 x 2 = 16 states in a 4 bit nibble. 2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 256 states in an 8 bit byte. With 4 bytes, 256 x 256 x 256 x 256 = 4,294,967,296 states. Yes, 4 billion is not enough numbers for all the world's ethernet ports, particularly as blocks are pre reserved. Occasionally IP numbers have not just 4 bytes (in IPV4 standard), but have 8 bytes (in IPV6 standard, that is about 16 billion billion. but not in wide use yet. ) A MAC (MAchine) number is actually: - not one per machine - but 1 per ethernet interface (so a router would sometimes have 4, & my gateway PCs have 2: 1 for internal net, & 1 to router) - MAC number is more like a chassis number embossed on a car, actually, if individual windows on a car had individual different serial numbers on each window, that would be an exact analogy. - MAC numbers are very hard if not impossible to change, dependent on hardware, so used for licensing. - MAC is a 6 byte number in hexa-decimal arithmetic (base 16), In decimal base 10 we use digits in range 0 1 2 3 4 5 6 7 8 9 In hexadecimal base 16 we use digits in range 0 1 2 3 4 5 6 7 8 9 A B C D E F (A=10, B=11, C=12, D=13, E=14, F=15) Each of the 6 bytes is comprised of 2 nibbles, Each nibble is comprises 4 bits, So you see some numbers like: 05:7A:B9:34:CD:E8 There are 256 x 256 x 256 x 256 x 256 x 256 = 281,474,976,710,656