Subnetting Chart?

UsualSuspect7

Do you guys actually utilize subnetting charts on the exam? I was told using this particular feature having a few columns such as Mask, CIDr, Subnet, etc ... and simply referencing the chart would be better then actually just "doing the math".

GDaines

It's a lot quicker to look at a chart than to count on your fingers. For example, a /20 Class B network, what's the subnet mask and block size?

On the chart below just remember for a class B the default is /16 so we have 4 extra bits. Count 4 in from the left using the top box and you can quickly see the subnet mask will be 255.255.240.0 giving a block size of 16. Now you can use the bottom box rather than having to count 16, 32, 48 which isn't so easy to remain accurate as you get to the other end and this way you don't forget that the address blocks end one before the next 16, so 0-15, 16-31, 32-47. Easy!

And just to round it off I include the powers of 2 for answering those 'How many subnets and how many hosts do you get' questions. Using the above example there are 4 extra network bits, so 4 to the power of 2 = 16 subnets. And that leaves us with 12 host bits so 2 to the power of 12 = 4096 less the network and broadcast addresses so there are 4094 possible host addresses. If you can answer 2^12=4096 in less than the two seconds it took me to look at the chart then you're too smart to need to write anything down...



I've not taken the exam yet so don't know if there's too much information, or if I have to include more, but I do know using this I can answer subnetting questions far more quickly, just have to keep reminding myself to deduct 2 from the number of hosts each time. Feedback welcome.

Subnetting helper.jpg