240 mask

Sorry, I know this will be flamed but I am finding conflicting information on this question throughout the internet.

How many subnets / hosts can you obtain from a /240 mask?

1. 14 subnets / 14 hosts
2. 14 subnets / 16 hosts

Thanks in advance,

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Comments

fualker

16 subnets with 14 hosts each

Now you have more conflicting information. Guess you'll have to verify this by learning the math behind subnetting.

ja5983

i may be wrong, but i have learned the 2^bits taken-2 formula and i would think that it would be (witha class c address)

14/14 since 240 is 4 bits taken away leaving 4 bits for host so..

2^4-2 = 14 subnets
2^4-2 = 14 hosts

it seems that you did
2^4 = 16 for subnets
and then
2^4-2 for 14 hosts

is the 2^(bits taken)-2 only for hosts and for subnets its 2^(bits taken) =?

chemaster

I realize that you substract 2 if you are not considering the zero - subnets etc.

However, how does Cisco want us to do this?

Thanks again,

fualker

The reason you take 2 off the hosts is this:

subnet 1 is adress 192.168.0.0-192.168.1.15

but 192.168.0.0 is the network addreass and 192.168.1.15 is the broadcast address, so those are not usable leaving 192.168.0.1-192.168.0.14 or 14 uasable addresses.

there are a total of 16 subnet *16 ip address = 255(starting from 0)

16 subnets with 14 ip's a piece

chemaster

many thanks

ja5983

so you are saying dont take 2 off of the subnet?

I just read CCNA for dummies and mike myers passport CCNA and the subnet formula in both books (the one that they say is for the ccna) is

2^(number of bits taken for subnet)-2

and

2^(number of bits left for host)-2

making 240 - 14subnet/14host

they the CCNA for dummies also goes on to state that some places including some MS tests and older CCNA tests do not use the -2, which is where you may have gotten 16/14 from....

dshevnock

I have noticed in my studies that some questions will ask about "how many subnets" or "how many usable subnets". For the "how many subnets" questions, don't subtract the 2 (that will give you all the subnets including the subnet address and the broadcast address). For the "how many usable subnets" subtract the two (that will eliminate the subnet address and the broadcast address leaving only addresses you can use for hosts). That is how I tried to remember it and it has faired pretty good for me.

dmaftei

chemaster wrote:

I realize that you substract 2 if you are not considering the zero - subnets etc.

However, how does Cisco want us to do this?

Thanks again,

Last time I checked Cisco wanted CCNA test takers to drop the first (all-zeros) and last (all-ones) subnets, so your answer should be 14. For higher level exams, as well as in real life, you'd have 16 subnets.

As for the number of hosts per subnet, you'd always subtract 2, as a previous poster explained.