VLSM is very confusing.
What you need to do firstly is organize your subnets by size largest first
1,000,000
500,000
250,000 and etc
Then you need to work out how what power of 2 is just greater than 1,000,000
In the example above you can see it is 2^20 .
The 20 is significant because this is the number of Hosts bit you need for this subnet.
Your given base network is 100.80 /12 which looks like in binary
11111111.0101
Subnet 0
2^20 -2 gives 1,048,574 hosts which is enough for 1,000,000 hosts
valid host addresses 11111111.01010000.00000000.00000001 to 11111111.01011111.11111111.11111110
Your next subnet add binary 1
11111111.0110
how many hosts?
need to find 2 power of just bigger than 500,000 which in this case is 2^19 =524,288 -2 = 524,286 which is enough for 500,000 hosts
Hope this helps you get on with it.
Peter
There are worlds out there where the sky is burning, where the sea's asleep and the rivers dream, people made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice and somewhere else the tea is getting cold" Dr Who
