Class B subnet with Class A IP?

CCNA_2007

I think maybe I missed something.

If we have a class A IP address 12.0.0.0 and a class B + subnet mask of 255.255.240.0, doesn't that mean that the first 16 most significant bits are reserved for the network and not the hosts?

Otherwise, why would we use a default class B mask against a class A address?

If I understand it correctly, because of the subnet mask, then the first two most siginificant octets refer to the network, and the last two refer to subnetting and hosts.

But the 'correct' answer to the question shows 24 bits for subnetting and hosts ( 4096 possible subnets, with 4094 possible hosts per subnet)

EDIT: obviously a beginner here! Looking for the simplest explanation.

EdTheLad

You have a class A address 12.0.0.0 and a mask 255.255.240.0, a class A address uses a natural mask 255.0.0.0 , so you have and extra 12 bits used as the subnet.Why are you talking about class B?
So 8 reserved + 12 subnetting = 20 bits reserved for the network, 32- 20 = 12 , 2^12 = 4096
Hosts = 2^12-2 = 4094

CCNA_2007

EdTheLad wrote:

You have a class A address 12.0.0.0 and a mask 255.255.240.0, a class A address uses a natural mask 255.0.0.0 , so you have and extra 12 bits used as the subnet.Why are you talking about class B?
So 8 reserved + 12 subnetting = 20 bits reserved for the network, 32- 20 = 12 , 2^12 = 4096
Hosts = 2^12-2 = 4094

Thanks Ed. I'm starting to see that the example doesn't use a natural class A mask because the subnet includes bits in the 3rd octet?

The subnet mask is telling us that there are 12 bits in the subnet portion.

(xxxx.255.240.xxxx) and that only makes sense when considering that the IP address is a class A address. Hope I got it right.

EdTheLad

A class A address will at a minimum have an 8 bit mask, anything extra is a subnet bit.
A class B address will have at a minimum a 16 bit mask anything extra is a subnet bit.
A class C address will have at a minimum a 24 bit mask anything extra will be a subnet bit.
Getting the picture? When you get comfortable with this idea and can subnet with ease you will move to summarization which moves the mask to the left of the natural boundary, but at the moment your working on subnetting so get those rules into your head

CCNA_2007

EdTheLad wrote:

A class A address will at a minimum have an 8 bit mask, anything extra is a subnet bit.
A class B address will have at a minimum a 16 bit mask anything extra is a subnet bit.
A class C address will have at a minimum a 24 bit mask anything extra will be a subnet bit.
Getting the picture? When you get comfortable with this idea and can subnet with ease you will move to summarization which moves the mask to the left of the natural boundary, but at the moment your working on subnetting so get those rules into your head

EdTheLad, thank you for the input.

Let me ask you then about the function of the mask against a class A address.

If the class of the IP address dictates the counting of all other bits regardless of subnet mask, then the mask is more about defining the range of hosts than about defining the subnets and hosts?

In other words, if we have a class A IP address, that in itself tells us ( without subnet mask), that we have 24 bits left over to define subnets and hosts....right?

So if that is true, could it be correct to interpret the subnet mask as defining the number of hosts, and could we from there derive the number of subnets?

In other words (again, and sorry, and bear with me!)....if we start with a class A address, then why have a mask that contains 255 in the second octet?

Maybe I am troubled over something that is sort of arbitrary. If the IP address is class A, then the mask in every other octet could define the number of hosts first, and then the number of subnets....or the other way around?

I guess I'm asking why we have to figure subnets first rather than hosts first. I think we can figure the number of hosts first if we want to.

Thanks Ed.

EdTheLad

The 255 in the second octet tells you 8 bits extra are being used by the network i.e. subnet bits.
It's really easy once it clicks, you can work out the subnets or the hosts first, its up to you, they both share the same bit space.

CCNA_2007

EdTheLad wrote:

The 255 in the second octet tells you 8 bits extra are being used by the network i.e. subnet bits.
It's really easy once it clicks, you can work out the subnets or the hosts first, its up to you, they both share the same bit space.

OK thanks Ed, I see it much better now.

network bits aside, the remainder of the bits are split between subnets and hosts.

super.