Problem with VLSM

Rushlan

Question:

IP : 172.16.0.0/20
User LAN has 520 hosts , Server LAN has 12 hosts
Sub-net the given network , assign the first usable sub-net to User LAN while wasting the fewest address.
Sub-net further the 3rd usable sub-net, and assign the 7th usable sub-net to Server LAN.

My workings:

2^10 = 1024, which is in the 6th position (4) of the 3rd octet.

First usable network ranges from 172.16.0.0 to 172.16.3.255
The next network begins with 172.16.4.0 and the sub-net mask is 255.255.252.0

Please correct me if I am wrong so far.

Sub-netting the 3rd usable sub-net for Server LAN ( this is where I am a bit confused )

12 hosts required , so 2^4 = 16, I increment the network by 16 in the last octet

172.16.4.16 should be the 5th sub-net ? and if I increment by 16 ...

172.16.4.32
172.16.4.64
172.16.4.96 ............> this the 7th usable sub-net ?

Rushlan

I shall be much grateful if you can please post any link where I can learn about this further.
Thanks a lot.