Zero Subnet

wbosher

I've read a lot of posts on the Zero Subnet but I'm just getting more and more confused. If the zero subnet can't be used, is this just the first subnet in the range? For example...

192.168.1.0/27 would mean the ranges are..

192.168.1.0 - 31
192.168.1.32 - 63
192.168.1..64 - 95
192.168.1..96 - 127....

Doest this mean the the range from 192.168.1.0 - 31 can't be used?

AlanJames

The zero subnet can be used, you need to count it in cisco exams as well unless they state otherwise, which they normally don't

wbosher

I understand that they can be used unless otherwise specified. What I want to know is am I correct in saying that this first range is the zero subnet? I'm confused as to what exactly the zero subnet is.

iworms

You're right, 192.168.1.0 -- 31 (/27) is subnet zero. This is because your subnet address is 192.168.1.0, which coincides with the class C network address 192.168.1.0. Other subnets don't have this problem.

Subnet zero is in reference to classful networks, so all you need is find the one subnet that has a subnet address identical to the classful network address. For example, 10.1.1.0/26 is not subnet zero, because the class A network address is 10.0.0.0, so 10.0.0.0/26 is subnet zero.

wbosher

OK, I think I've got it. So using a class B example of 172.16.0.0/26, the zero subnet (assuming we can't use the zero subnet) would be 172.16.0.0 - 63 and would be unusable, but we could use 172.16.1.0 - 63. Am I right?

iworms

Right, we can't use 172.16.0.0--63. For your /26 example, we can use 172.16.0.64--127 all the way up to 172.16.255.192--255.

Just for fun, 172.16.0.0/23: if subnet zero not allowed, then 172.16.0.0--172.16.1.255 cannot be used. The first valid range is 172.16.2.0--172.16.3.255.

wbosher

Sweet...think I've got it. Cheers mate.

Funky_child

iworms wrote: »

You're right, 192.168.1.0 -- 31 (/27) is subnet zero. This is because your subnet address is 192.168.1.0, which coincides with the class C network address 192.168.1.0. Other subnets don't have this problem.

Subnet zero is in reference to classful networks, so all you need is find the one subnet that has a subnet address identical to the classful network address. For example, 10.1.1.0/26 is not subnet zero, because the class A network address is 10.0.0.0, so 10.0.0.0/26 is subnet zero.

Does for address 192.168.2.0/28 the subnet-zero addressess going from 192.168.2.1 - 192.168.2.15 or from 192.168.1.1 - 192.168.1.15 ?

Does subnet - zero always starts from begin of class or?

thnx!

thehourman

If the command no ip subnet-zero is being used. Can use the broadcast address?

fly351

thehourman wrote: »

If the command no ip subnet-zero is being used. Can use the broadcast address?

Your question isn't very clear

blackninja

Before Cisco introduced the Subnet Zero command, you would lose the first and last subnets, just like you would when you -2 for the hosts (network & broadcast addresses)

E.G.

Without Subnet Zero command:
192.168.0.0 /28
192.168.0.0 - 15 - CAN'T USE
192.168.0.16 - 31
192.168.0.32 - 47
.
.
192.168.0.224 - 239
192.168.0.240 - 255 - CAN'T USE

With Subnet Zero command:
192.168.0.0 /28
192.168.0.0 - 15 - CAN USE
192.168.0.16 - 31
192.168.0.32 - 47
.
.
192.168.0.224 - 239
192.168.0.240 - 255 - CAN USE

thehourman

blackninja wrote: »

Before Cisco introduced the Subnet Zero command, you would lose the first and last subnets, just like you would when you -2 for the hosts (network & broadcast addresses)

E.G.

Without Subnet Zero command:
192.168.0.0 /28
192.168.0.0 - 15 - CAN'T USE
192.168.0.16 - 31
192.168.0.32 - 47
.
.
192.168.0.224 - 239
192.168.0.240 - 255 - CAN'T USE

With Subnet Zero command:
192.168.0.0 /28
192.168.0.0 - 15 - CAN USE
192.168.0.16 - 31
192.168.0.32 - 47
.
.
192.168.0.224 - 239
192.168.0.240 - 255 - CAN USE

Ok this answers my question. Thanks