Hi, well i have spent the past 6 months studying for my CCNA, so i pretty much know subnetting inside out. I have learn't alot about how networks work in this time. I am not sure if that answer is exactly right,

but if you do the math, a /12 means 255.240.0.0....11111111.11110000.00000000.00000000

Since the second octet is a 240, that means an increment of 16 (256-240). So this would mean subnets of 172.0.0.0 , 172.16.0.0 , 172.32.0.0 and so on

So the private address range for class B networks falls in the 172.16.0.0 subnet...172.16.0.1 - 172.31.255.255

Correct me if i am wrong

RFC1918 says:

QUOTE

...

172.16.0.0 - 172.31.255.255 (172.16/12 prefix)...

...

the second block is a set of **16 contiguous**

class B network numbers

...

I bolded out what actually confuses me the most.

If we use 255.240.0.0 mask then block size is

**16**, hence network numbers are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240 (totaling 16 networks).

But interval 172.16.0.0 - 172.31.255.255 covers only one of listed networks!

In this interval according to mask

**255.**__240__.0.0 host bits are used in second octet - from bit

**3 to 0**, that makes up the "

**31**" (16 - network number, and 1111b = 15d, 16 + 15 = 31).

So, for "16 contiguous class B network numbers" to be true the network mask should be

**255.**__255__.0.0!

But what does "

**/12 prefix**" mean??!

Guys, I'm totally lost. ((