Questions about Imran's video on subnetting

Oh! Gotcha.

First part of the answer:

Why split the B group in half?

TL;DR - Because IPv4 is 32 bits so 1/2 of the range is 16 bits.

There are 32 bits in an IPv4 address. The class A address would take up 25% of the bits for the network with the rest remaining as hosts. The B takes up 50% of the address bits as Network number and the rest as hosts and C is 75% as Network numbers.

So with the B you have:

NNNNNNNN.NNNNHHHH.HHHHHHHH.HHHHHHHH
N = Network ID bits
H = Host number bits

Part two of the answer:

Why do we start at 172.16.x.x?

The first answer I gave answers why we use 172 - because of the 1010 0001 pattern.

But why .16.x.x?

So if we look at the second octet the actual values that the bits represent are:

128 64 32 16 8 4 2 1 <----- "Place value" N N N N H H H H

So when they set aside ranges for private not-to-be routed externally IPs they decided to make the B range equal to 1/2 the address space as network 1/2 as host. But, there was a problem - some older equipment (VERY old equipment) didn't know how to deal with subnetworks (They did fine with the A and C because both use a full octet) so it was decided that the "B" private class would begin with the first network that wasn't in the Zero Subnet - that is to say the second available network so 172.0.0.0 as a network number was out of play so it increments to the next one 172.16.0.0 as the private B range.

/r/ccna Thread Parent