# Subnetting math question.

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• 25th February 2008, 09:16 PM
nigel6666
Subnetting math question.
Hi just a quick question i'm studying for my CCNA and have been trying to wrap my head around subnetting, as part of my study i've been pulling questions from online and one of the questions was :

identify the 125th subnet on a 10.0.0.0/18 (255.255.192.0)

and for hours now i've been trying to work out a quick way to work out the 125th but have no idea of easy ways.

i ended up doing:

10.0.0.0 is subnet 0
10.0.64.0 is subnet 1
10.0.128.0 is subnet 2
10.0.192.0 is subnet 3
10.1.0.0 is subnet 4

then i went "if subnet 4 is 10.1.0.0 then subnet 8 is 10.2.0.0 and so on"

and worked my way to subnet 125.

and my answer was 10.31.64.0

no idea if my answers are correct or not but can anyone point me in the right direction of an easy way to work out identifying specific subnet numbers?
• 25th February 2008, 10:20 PM
clarky2k3
Yes you are indeed correct.
10.0.0.0 Private Class A address /18 or netmask 255.255.192.0
Gives you 1024 subnets with 16,382 hosts per subnet.
Subnet 125 would give you a network address of 10.31.64.0 and the broadcast address of 10.31.127.255.
• 25th February 2008, 10:48 PM
nigel6666
Thanks for the reply, is there an easyer way to work out a specific subnet number? you seem to have done it alot easyer than i did! :)

cheers
• 26th February 2008, 08:02 AM
torledo
Quote:

Originally Posted by nigel6666
Thanks for the reply, is there an easyer way to work out a specific subnet number? you seem to have done it alot easyer than i did! :)

cheers

nigel

not sure about an easy way, but the way i work these out is by using the method used by my ccna instructor, who was a particularly bright man...

draw out the four octets on a piece of paper

xxxxxxxx xxxxxxxx xxxxxxxx xxxxxxxx

draw a dotted line down /18 to distinuish between the network portion and host portion of the address space..

xxxxxxxx xxxxxxxx xx|xxxxxx xxxxxxxx

working back from the right-most digit on network portion assign the following 1, 2, 4, 8, 16, 32, 64 etc. as below, we'll not need 128 as that exceeds the 125 you're trying to work out.

64 32 16 8 4 | 2 1

1 1 1 1 1 o 1

now simply 1 the numbers that are used to make up 125 (the 125th subnet you're looking for) as above

Then just adjust it by working in the correct order, so it then becomes ;

16 8 4 2 1 | 128 64

1 1 1 1 1 o 1

And now we just add up the ones in all octets

so the first octet is still 8 + 2 = 10
the second octed is now 16+8+4+2+1= 31
the third octet is just 64 = 64
the fourth octet is the host portion entirely so that's all = 0

10.31.64.0

for the broadcast just 1 the host portion so that becomes.....

10.31.127.255

i'm sure someones going to point out a mistake in my workings or show you an easier way, but it was a lot quicker working it out using this method than you'd think.
• 27th February 2008, 08:42 PM
nigel6666
subnetting
while we are on the subject of subnetting and trying to overload my brain
i dont supose you have an easy way or example for vlsm :)

the next section of the 125th subnet question wants to vlsm for 128 subnets my calcs ended up like this:
so we start from 10.31.64.0

which is
00001010.00011111.01000000.00000000

so the original mask is 255.255.192.0 (/18)

so we have used 10 bits for subnetting
00001010.00011111.01000000.00000000

so to futher subnet for 128 subnets i borrow a further 7 bits because 7 to power of 2 is 128 which is a mask of 255.255.255.128 (/26)

so now i have
00001010.00011111.01000000.00000000

so i took the
00001010.00011111.01000000.00000000

turned all the borrowed bits on and end up with
64+32+16+8+4+2+1=127

so my first subnet is
10.31.64.0 (Subnet 0)
10.31.64.128 (subnet 1)
10.31.65.0 (subnet 2)

and so on and i worked out my last subnet as being

10.31.127.128 (subnet 128)

i think the math is correct. but i would like an easyer way to remember such as the way u explained earlyer :)

not sure if u or anyone can help but it would be majorly appreciated!