In my previous post we started talking about subnetting IP addresses using mathematical formulars and then I wrote something’s like 2 to the power of x (2*) equal number of networks, while 2 to the power of x minus 2 equal the number of valid hosts on our network (2*)-2
If your are just refreshing your memory you recall that the 2 we minus is the network address and the broadcast address on our network but in case you’re new to networking it is important you take note of this in CCNA exam and in real life scenario.
With all these in place I think is time we dive into some practical tutorials and see exactly what we are talking about here.
Lets assume we have this
class B address 220.127.116.11
with it’s subnet mask of 255.255.0.0
Now the organization we are working for, has grown so big and they want us to break this network into one hundred networks (100 networks).
We will draw our chart here:
128 64 32 16 8 4 2 1 in case you don’ know these are just powers of 2
100 in binary is equal 100 = 01100100 this means that it take nothing less than 7 bits to get to the number 100, that is our first step to success.
Secondly we will line up our subnet mask in binary once again
255.255.0 = 11111111.11111111.00000000.00000000
We are adding more networks so we move 7bits from left to right and this we turn these 7 bits on and then we will have our new subnet mask
So then 255.255.1111111.0.00000000 = 255.255.254.0
Our lowest network range which is our increment is the last network increment which is 2
We will use 2 to find our network increment.
So we have
Note that in between these network ranges we are going to run into 18.104.22.168,
1505.3.0 and so on the question is are they valid IP”s?
Let’s find out
(2*) where x = number of networks
(2^7) = 128 networks
(2^7)-2 =126 host on each networks so they are eventually valid host IP’s and can be assigned to computers, and phones.
So that is how to use mathematical formular in subnetting.
Final piece here is that once your able to figure out how many bit it will take to get the host or network you want create you can skip all these long processes and do it mathematically.