Easy subnet question

I'm working on answering subnetting questions faster but how do you answer these types of questions fast without adding up 16 until you get up to the higher numbers?

On what subnet is the IP address 222.17.32.244/28 found?

222.17.32.240/28
222.17.32.192/28
222.17.32.224/28
222.17.32.216/28

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Comments

mens

fivkxpert wrote: »

I'm working on answering subnetting questions faster but how do you answer these types of questions fast without adding up 16 until you get up to the higher numbers?

On what subnet is the IP address 222.17.32.244/28 found?

222.17.32.240/28
222.17.32.192/28
222.17.32.224/28
222.17.32.216/28

when you know it's a block of 16, you can just subtract 16 from 256 and find the subnet number. The first answer is correct.

You don't necessarily need to start at 0, you can start counting from 128 or 192 or something as well.

Neeko

You can work back from 256. Or just get a feel for what is the closest multiple to the IP you have been given. For example if you were given 200.10.10.150 /29. What number do we know is near to that, no matter what the power number is? 128. In this case we are using multiples of 8 so can just add 8 a couple of times to find our subnet.

If you stick to what you know are safe numbers, 128, 192, 224, 240 etc you can give yourself something to work from. Eventually you should be able to figure it out in a matter of seconds as the maths does the work for us.

blackninja

Neeko wrote: »

If you stick to what you know are safe numbers, 128, 192, 224, 240 etc you can give yourself something to work from. Eventually you should be able to figure it out in a matter of seconds as the maths does the work for us.

This what I do and can do most questions in around 15-20 secs.

To find the multiples (no magic numbers or tables): just to go off the mask.

mask 128 = multiple 128, 192 = 64, 224 = 32, 240 = 16, 248 = 8, 252 = 4 and 254 = 2

Subnetting is all about practice, practice and more practice.

LBC90805

Yup, a lot of it is practicing and internalizing the numbers that make up the multiples of the mask.

I was lucky from the get go because I tutored algebra for over ten years and knew how to count in binary since the sixth grade. Also knowing what the double of the previous number was.

blackninja

LBC90805 wrote: »

and knew how to count in binary since the sixth grade.

Smart arse - lol

Forsaken_GA

fivkxpert wrote: »

I'm working on answering subnetting questions faster but how do you answer these types of questions fast without adding up 16 until you get up to the higher numbers?

On what subnet is the IP address 222.17.32.244/28 found?

222.17.32.240/28
222.17.32.192/28
222.17.32.224/28
222.17.32.216/28

/28 = 255.255.255.240

256 - 240 = 16, this is your range

Incrementing on the 4th octet, so take that number and divide by the range

244 / 16 = 15.25. Drop the fraction, and multiply the whole number by the range

15 * 16 = 240, this is the lower boundary for the subnet

222.17.32.240/28

captobvious

Forsaken_GA wrote: »

/28 = 255.255.255.240

256 - 240 = 16, this is your range

Incrementing on the 4th octet, so take that number and divide by the range

244 / 16 = 15.25. Drop the fraction, and multiply the whole number by the range

15 * 16 = 240, this is the lower boundary for the subnet

222.17.32.240/28

/28 is in the fourth octet so 32 - 28 = 4

2^4 = 16 this is your range

continue on his path

Forsaken_GA

captobvious wrote: »

/28 is in the fourth octet so 32 - 28 = 4

2^4 = 16 this is your range

continue on his path

Right, use whatever method you prefer to find the range, it doesn't matter, just whatever works for you. I'm in the habit of translating CIDR notation out into dotted decimal just because so many IOS commands require it, or the inverse mask

blackninja

Forsaken_GA wrote: »

/28 = 255.255.255.240

256 - 240 = 16, this is your range

Incrementing on the 4th octet, so take that number and divide by the range

244 / 16 = 15.25. Drop the fraction, and multiply the whole number by the range

15 * 16 = 240, this is the lower boundary for the subnet

222.17.32.240/28

This method is OK if you have a calulator and they not allowed in the exam room

If you can divide 244/16 and come to 15.25, in your head then I think mensa is calling you

For high number start at 255 and minus the muliple until you get below 244.
For a low number keep adding the muliple until just below the number

Forget magic numbers:

blackninja wrote: »

To find the multiples (no magic numbers or tables): just to go off the mask.

mask 128 = multiple 128, 192 = 64, 224 = 32, 240 = 16, 248 = 8, 252 = 4 and 254 = 2

Forsaken_GA

blackninja wrote: »

This method is OK if you have a calulator and they not allowed in the exam room

No, but you do get something eraseable to write on, and a marker to write with. I refuse to believe that education has fallen so far behind that people are incapable of doing long division.

If you can divide 244/16 and come to 15.25, in your head then I think mensa is calling you

And yeah, I can do that division in my head, but I wouldn't need to go all the way to 15.25, just to 15, after that, it doesn't matter anymore.

Forget magic numbers:

Well, to each their own - I will point out this though.

The point of subnetting exam questions is to learn how to subnet, not to learn how to answer subnetting questions. Knowing the significance of the magic numbers and *why* they're the magic numbers goes a long way towards solidifying the understanding. In the end, there are only 3 magic numbers when it comes to ip 4 addressing... 2, 8, and 32. Knowing why is at the core of understanding how addressing works.

kryolla

wow I haven't heard of magic numbers since I passed my NA in 2005