11-11-2011 12:54 AM
Hello Everyone,
What is the formula or mathimatical operation to determine whether two addresses are on the same subnet or not?
I know that I can use AND operation to do this but this is time consuming, also I know the formula in MS Press book which is :
f1 = [K1 / (256-d)] - any reminder
f2 = [K2 / (256-d)] - any reminder
Where K1 and K2 are the respective value of the octet in each IP address whose position corresponds to d
and "d" is the modrange value octet in the subnet mask.
didn't get it right
lets take this example to understand:
I need to determine whether these rwo IP addresses are in the same subnet
192.168.0.180 180 is K1
192.168.0.192 192 is K2
with net massk of 255.255.255.224 224 is d
using this information we will see
f1 = [180 / (256-224)] = 180 / 32 = 5.625
f2 = [192 / (256-224)] = 180 / 32 = 6
f1 is not equal to f2 and so they are not in the same subnet!
Well this formula works sometimes but others not!
now lets do the traditional way of determining whether these rwo IP addresses are in the same subnet:
Netmask : 255.255.255.224
Binary : 11111111 11111111 11111111 11110000
The increment value is 16 therefore:
192.168.0.16 192.168.0.31
192.168.0.32 192.168.0.47
192.168.0.48 192.168.0.63
192.168.0.64 192.168.0.79
192.168.0.80 192.168.0.95
192.168.0.96 192.168.0.111
192.168.0.112 192.168.0.127
192.168.0.128 192.168.0.143
192.168.0.144 192.168.0.159
192.168.0.160 192.168.0.175
192.168.0.176 192.168.0.191 here is the first IP Subnet ( 180 )
192.168.0.192 192.168.0.207 here is the secons IP Subnet and it is the network ID itself ( 192 )
192.168.0.208 192.168.0.223
192.168.0.224 192.168.0.239
Now we know that they are not in the same subnet!
The Question IS, How can I jump directly to the desired range without writing all of these ranges to get to the desired range ?????
I saw a vedio before and the guy said that I can use the number and make it power of 10 to jump directly to the Range ??? but I can't find the Video
Does anyone has idea how to do this
Thank you very much
Solved! Go to Solution.
11-11-2011 02:41 AM
wouldn't this give you what you want:
if the devision has a rest (module not eqals 0) # e.g. 180/32 = 5.625
take just the integer and multiply it with [256 - d] # 5 * 32 = 160
it gives you the network directly: 192.168.0.160
but you made a little mistake - just a bit-swap..
Netmask : 255.255.255.224
is
Binary : 11111111 11111111 11111111 11100000
and not
Binary : 11111111 11111111 11111111 11110000
therefore, the increment value is 32 not 16 - and that is what you get when you subtract 224 from 256:
256 - 224 = 32 (the number of addresses for each subnet)
if the modulo equals 0 you have the first IP of the network directly;
11-11-2011 02:41 AM
wouldn't this give you what you want:
if the devision has a rest (module not eqals 0) # e.g. 180/32 = 5.625
take just the integer and multiply it with [256 - d] # 5 * 32 = 160
it gives you the network directly: 192.168.0.160
but you made a little mistake - just a bit-swap..
Netmask : 255.255.255.224
is
Binary : 11111111 11111111 11111111 11100000
and not
Binary : 11111111 11111111 11111111 11110000
therefore, the increment value is 32 not 16 - and that is what you get when you subtract 224 from 256:
256 - 224 = 32 (the number of addresses for each subnet)
if the modulo equals 0 you have the first IP of the network directly;
11-11-2011 03:30 AM
OMG it's not a small mistake I did, it's BIG which could ruin the whole question lol
sorry about that
Very good Martin it is easier now for me
Thanks
11-11-2011 04:38 AM
In addition to Martin's correct answer regarding appllication of the formula you cited, you may also want to consider a tool that you can run for this. Solarwinds offers a free (ad-supported) version of their Subnet Calculator that you can download to your desktop. Here is a link to it. It's also included (without ads) in their paid Engineer's Toolset product.
Of course there are also web sites with tables and such.
11-11-2011 05:59 AM
Thanks a lot mklemovitch
The software is awesome but I needed this information for the Cisco exam and Microsoft infrastructure exam too, therefore the calculator would be useful for me only as a helper to chk my results but it's very helpful.
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