Subnetting question. Please help!

Guys I came across this question and don't understand how they got the answer for the number of subnets. I do understand how they got that number of hosts per subnet though.

Question: How many subnets and hosts per subnet can you get from the network 10.0.0.0 255.255.240.0?

Answer: 4096 subnets and 4094 hosts

Okay, I to find the answer to this I would usually proceed as follows:

By looking at the subnet mask I know that 4 bits were borrowed. Now to find the number of subnets you would take: 2 to the power of 4= 16, right?

Then I would take the number of bits availble for the hosts (which is 12) and do:
2 to the power of 12= 4096-2 (for the subnet and broadcast) and come up with 4094.

Any insight into this would be greatly appreciated!!
Thanks Guys!

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Comments

henserlj

I was looking at what you wrote and wihtout doing any math...why do you say only 4 bits are borrowed? I see a class A address with a 255.255.240.0 subnet mask. To me that would say 20 bits were borrowed not 4...maybe that is where your math has gone wrong.

Just thought I would put in my 2 cents

henserlj

Sorry...correction...12 bits were borrowed.

That is why I stayed away form the math at first

remyforbes777

10.x.x.x is a class A subnet so it has a default subnet mask of 255.0.0.0 so if they have given you a subnet mask of 255.255.240.0 that means that 12 bits were borrowed for the subnets. Do the math from there.

Victorious

because from what I understand according to the subnet mask the interesting octet is: 240.
In binary= 11110000 so, I think 4 bits were borrowed. What do you think?

Victorious

remyforbes777 wrote:

10.x.x.x is a class A subnet so it has a default subnet mask of 255.0.0.0 so if they have given you a subnet mask of 255.255.240.0 that means that 12 bits were borrowed for the subnets. Do the math from there.

Ooohhhhh! I see. Now I understand!! Cool! It makes sense. Thanks guys!!

remyforbes777

Victorious wrote:

because from what I understand according to the subnet mask the interesting octet is: 240.
In binary= 11110000 so, I think 4 bits were borrowed. What do you think?

I think you are confused about subnet masks. You have to think default Class A, Class B and Class C subnets. When you get a 10.x.x.x that is a class A address which has a default subnet of 255.0.0.0. You have to borrow bits from the second and third octet to get your 255.255.240 subnet.

henserlj

You have to start with the default subnet mask
255.0.0.0 or
11111111.00000000.0000000.0000000
then you have to borrow from the host portion which is the octets containing all zeros
255.255.240.0 or
11111111.11111111.11110000.00000000

You can see you have borrowed from the host field 12 bits to create the subnet field that I bolded.

So a quick look at this would be you would get the same number of subnets as you would host addresses (minus the 2 for network and broadcast addresses)

Hope that explains it...if not there are many other people on here much smarter than I

Victorious

henserlj wrote:

You have to start with the default subnet mask
255.0.0.0 or
11111111.00000000.0000000.0000000
then you have to borrow from the host portion which is the octets containing all zeros
255.255.240.0 or
11111111.11111111.11110000.00000000

Hey dude!
Really appreciate your insight. I understand now......Thanks for your time and help!!

henserlj

No problem...good luck with your studies.

Victorious

And to you as well my brother.