I guess this is fairly simple.
You need to know a small thing about how the addressing and transmission works. (Correct me if i am wrong)
Heard of multiplexer?
This is an essential component of any transmission. why?
The above is a simple 2:1 (2 input channel-1 output channel) Multiplexer.
We know that in a computer, data needs to be constantly transferred,
1) from secondary storage to primary storage
2) The graphic content has to be processed by the GPU and sent to the display unit. etc.
I hope you get the vibe. There is always data being sent from one device to other internally. Now check out your Computer, It has only one bus cable for all this.
Our aim is to reduce the no. of cables being used. Here comes the Multiplexer,
In the above figure there are two sources of input, we use a single bit "sel" to find which source it is! That is, 1 bit is used to tell which source has it input being transmitted,
logical right? 1 bit can have two values 0 or 1.
0-1st sourse (means 1st source is transmitting data)
Now, how many bit you need for addressing 4 sources? 2 bits!
Similarly, 8 sources require 3 bits to uniquely identify it.
Thus using a multiplexer data from multiple sources can sent through 1 single channel.
Now comes the demultiplexer. Above is 1:2 Demultiplexer (1 channel input-2 channel output)
So out of the multiple sources, the "sel" or select gives the unique bit values to find which source is transmitting data. At the receiver end, a demultiplexer is used, wherein, it uses the "sel" bits to give the transmitted data to the necessary output device (could be a different output device, so "sel" may vary).
1) Doing this helps us in easy transmission which reduced complexity in terms of cables
2) Reduction in cost
3) Easy addressing of devices (as a multiple of 2). Each address is unique and 16 bit means 2^16 possible devices could be connected.
4) Not only devices, each bit/byte in the primary and secondary need to be addressed. Thus, 32 bits or 64 bits can address a large number of location/devices.
Thus, since we are dealing with binary values, everything is in powers of 2. So, no 6 or 17...etc.