# ccnp age old subnet formula question

Jul 14th, 2007

passed ccna, studying for ccnp starting with bsci. frustrated by the variable formulas (still) for calculating max number subnets. With CCNA you assumed 2^subnetbits - 2; unless they specifically told you a classless protocol was in play.

But what I'm reading now to prepare for ccnp says just use 2^subnetbits no "minus two". I'm using ciscopress book also Bryant advantage.

So I'm practicing on learn to subnet just to build some speed in preparation for exam and, they still deploy the -2 part of the forumula.

Has Cisco yet specified exactly what their 'standard' is???

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## Replies

Chris Deren Sat, 07/14/2007 - 11:39

You may be confusing number of subnets vs hosts per subnet.

Number of subnets = 2^subnetbits

Number of host in subnet = 2^subnetbits - 2 (default network, broadcast)

so, consider a Class A network 10.1.X.X/16

Number of subnets = 2^8=256

Number of host per subnet = 2^8-2=254

list of subnets:

10.0.X.X

10.1.X.X

....

...

10.255.X.X

for a total of 256

HTH,

Chris

Chris Deren Sat, 07/14/2007 - 11:53

Sorry, I goofed up on the number of hosts calculation:

2^16-2=65,534 in this example.

Chirs