Icnd2 - vlsm

20Euro

Hi everyone,

I am going to take the ICND2 exam next week and I spent lots of time practicing subnetting/VLSM.

I am getting pretty good but I still have trouble if the biggest subnet in the question is more than 128 hosts.

For example, say the subnet has 2000 hosts. I can easily figure out that this is a /21 but, how to I calculate (using pen and paper only) the range of such a large subnet! It gives me a lot of issues since I can't figure out what address space I am allowed to use for the next subnet.

Any tips would be greatly appreciated! Thank you in advance!

spd3432

Not sure I follow what you're saying about being able to calculate the next subnet. Do you have any problems with /24 --> /30 calculations? The only difference between /21 and /29 is you're using the 3rd octet vice the 4th ( 255.255.248.0 vice 255.255.255.248 ) so dealing with 172.16.x.x network

172.16.0.0 -> 172.16.7.255 (/21)
172.16.8.0 -> 172.16.15.255 (/21)

172.16.0.0 -> 172.16.0.7 (/29)
172.16.0.8 -> 172.16.0.15 (/29)

The more you play around with the numbers, the easier it is to see the patterns.


Good luck on the test.

Sean

andy4tech

Yea it should be /21 because the number of bits that will be turned ON, for 2000 hosts should be 11 ,you get 11 by finding the bits using binary charts 2048 ,1024,512,256,128,64,32,16,8,4,2,1.looking at this with 2000 hosts ,just look at the chart and see where you can easily place the2000 hosts into,since you cannot substract 2048 from 2000 but out of the chart you have to end counting the on bit at 1024 ,there turning the bits ON starting from your right that is from 1 and ends it on 1024 ,so this shows that we have 11bits on.so to get the cidr notation /21 which gives us the subnet mask of 255.255.248.0.Since we know that this is a class B subnet of 255.255.0.0 which is the same thing as 255.255.00000000.00000000 and counting from right to left to find the mask with the 11bits,it will be 255.255.00000000.00000000 255.255.11111000.00000000 255.255.248.0