subnet mask question

[dannan lin]

hi:

I was doing my subnet excise, i ran into a question.

the question was 172.16.10.0/22 , what is its network address ??

the answer is 172.16.8.0/22 .

why is that?

i thought 172.16.10.0 /22 's

first network was 172.16.10.0 - 172.16.13.255

I knew if the network is  172.16.8.0/22 , it would have 172.16.10.0 in between. but

my question is how to calculate 172.16.10.0/22 's network address ??

thanks in advance.

[ankugarg]

Convert 172.16.10.0 into binary..If some bit is set after first 22 bits,make that 0.The resulting address is network address.

[dannan lin]

thanks for your answer, but i am thinking if there is any other easy way to slove this question without looking into binary.

[joeadiutori]

when I see /22, that tells me the number of nodes in the subnet will be 4.  this means the subnet must start at 0, 4, 8, 12, 16, .....

this is why it doesnt start at .10, like you assumed.

the subnet is 172.16.8.0.0 thru 172.16.11.255.

[cadet alain]

Hi,

of course there is a quicker way to solve this. Just use the magic number trick:

172.16.10.0 /22  /22= 255.255.252.0 so magic number is 256-252=4 which is the subnet increment

first subnet: 172.16.0.0/22 because 0*4=0

second subnet: 172.16.4.0 because 0+4=4

third subnet: 172.16.8.0 because 4+4=8

next one : 172.16.12.0/22 because 8+4=12

so of course 172.16.10.0/22 is part of 172.16.8.0/22 subnet

Regards.

Alain.

[dannan lin]

thanks so much. you guys are great.