Let me preface this by advising you write some sort of version of this chart on your dry erase board before beginning the exam, so you are not scrambling to remember it while the clock is ticking down during the exam, and if you don’t understand this chart read about it below until you do!
While going through some practice tests, I realized how stagnant my subnetting had become, and the only thing I remember was writing everything out in binary which was a time killer both on the practice exam in definitely will be in the class room.
That being said, I stole the above graphic from Keith Bogart’s CCNA subnetting class (with his express permission), and wanted to post this up not only for my but anyone elses review how to come up with answers fast on exam day for questions regarding different subnets.
You may be presented with a question like “A carrier provides an IP Subnet of 220.127.116.11/24, and you must make x amount of subnets that provides for the most amount of users”
Given that we already know from CCNA studies, that /24 indicates that the first 3 octets are spoken for, and we can borrow bits from the host bits we are provided to create subnets, and any left over bits are our available host bits (minus 2 for network and broadcast address of course).
“From the right rearmost we discover our hosts”
This is a way of saying that we can start at a /32, and find how many bits we have entirely, then apply the formulas -2 for subnet / broadcast address as such:
/32 = 0 bits available
/31 = 2 bits available
/30 = 4 bits available
/29 = 8 bits available
/28 = 16 bits available
/27 = 32 bits available
/26 = 64 bits available
/25 = 128 bits available
/24 = 256 bits available
/23 = 512 bits available
/22 = 1028 bits available
Beyond that, I will let you continue to double the number, minus two bits for network / broadcast addresses. So you can count backwards from 32 and get the above values, but again I strongly recommend you write a chart out to reference quickly for subnetting on the dry erase board your provided, even if it’s just a tiny table in the corner of the sheet.
What this information does is given you three things: The number of available hosts, the subnet range of available hosts once you find the network number, and also the available number of subnets.
Number of Available Subnets
As seen in the chart, the subnet bits are counted from where the assigned mask stops, and grows exponentially by the power of two (fancy way of saying they double) as they go further into the “Host Bits” available.
So if we go with our example of 18.104.22.168/24 from our ISP, lets borrow 1 host bit from our 8 host bits available and see the networks we have:
22.214.171.124/25 = 126.96.36.199 – 188.8.131.52 (184.108.40.206 Broadcast)
150.500.100.128/25 = 220.127.116.11 – 18.104.22.168 (22.214.171.124 Broadcast)
Now if we were to borrow two host bits for our subnet mask, we get the following ranges:
126.96.36.199/26 = 188.8.131.52 – 184.108.40.206 (220.127.116.11 Broadcast)
18.104.22.168/26 = 22.214.171.124 – 126.96.36.199 (188.8.131.52 Broadcast)
184.108.40.206/26 = 220.127.116.11 – 18.104.22.168 (22.214.171.124 Broadcast)
126.96.36.199/26 = 188.8.131.52 – 184.108.40.206 (220.127.116.11 Broadcast)
And so on.
Now to find a network range from a single IP address, given the examples above, lets say we are given asked what the range is of 18.104.22.168, we can immediately skip the first 3 octets.
So we skip to the 4th octet, and break the IP address of 174 into binary along with the mask, and perform the boolean “AND” process to find our network number:
/26 (4th octet) = 11000000
.174 IP Addy = 10
That is actually as far as we need to go, because that is how far the IP matches the subnet mask, so this would be in the 22.214.171.124/26 network.
To explain why this works so quickly and easily, because there is only 4 possible subnets that can come from the /26 mask in respect to the 4th octet in binary:
00000000 = .0
01000000 = .64
10000000 = .128
11000000 = .192
Red being the host bits available for the network (except of course all 0’s and all 1’s for network / broadcast addresses).
However on the CCNP, you are more likely to encounter questions that require quickly identifying host ranges that do not overlap when looking at multiple network, so being able to identify that kind of information quickly is key.
For example, we don’t even need the IP address, if we are asked “X company requires at least 26 subnets with the most amount of hosts, how many hosts will be available per subnet if the ISP assigns X company 126.96.36.199/24?”
Going along the chart, we know we need to borrow 5 bits which will give us 32 subnets, meeting the requirement (/29) and we know that a /29 mask will leave us with 8 host bits – 2 for network and broadcast address.
So the network numbers in the above question / answer would be as follows:
And so on 🙂
That is all I will beat that horse to death about subnetting, just be very clear on the concept of borrowing network bits, and host ranges.
AND FOR ONE LAST TIME, BE SURE TO WRITE OUT SOME KIND OF CHART BEFORE YOU START THE CLOCK ON THE EXAM, UNLESS YOUR ARE A SUBNETTING GENIUS (AND EVEN THEN WRITE IT OUT BEFORE STARTING THE TIMER)!