What is similar about MAC address in different computers?

Answer 1

IP addresses are unique on a given network. Networks are divided into subnets. A Subnet is specified by the network mask. If you look at an ip address and network mask in binary (as machines do) it all makes more sense. A common network mask you will see in a private ip network is (in binary)

11111111 11111111 11111111 00000000

grouped 8 bits at a time for clarity

That is 255.255.255.0 look familiar?

The mask tells us that any address that is to be considered part of our network must match exactly wherever there is a binary 1 and can differ anywhere there is a binary 0. Thus the addresses:

11000000 10101000 00000000 00000001 (192.168.0.1)

11000000 10101000 00000000 00000010 (192.168.0.2)

11000000 10101000 00000000 00000011 (192.168.0.3)

are the same when we look in the columns that have binary 1 in the mask (ie the first 3 decimal numbers) and differ only in the columns where there is a binary 0.

An address like

11000000 10101000 00000001 00000010 (192.168.1.2)

differs in a column where there is a binary 1, so this address is not in our subnet.

Thus, to actually answer the question, the parts of the address that are the same in different computers (if they are on the same subnet) is determined by the network mask.

Answer 2

A MAC address is the hardware specific address of a network device (NIC in your computer, network connected devices like printers or routers). Each device is supposed to have a different hardware address. To ensure uniqueness, manufacturers are assigned blocks of address space to use for their products. A MAC address is usually quoted as 6 pairs of hexadecimal numbers, often with a colon or a dash separating them. The first 3 pairs are a block for a given manufacturer. You can see a list at http://www.cavebear.com/archive/cavebear/Ethernet/vendor.html that shows a lot of them. Any device manufactured by Telebit corporation for example, would start with 00C033. Consequently, the part that is the same in different computers is the beginning, but only if they are made by the same manufacturer.