Subnet zero concept confusion

palit

The more i read about subnet zero more I am confused though I can solve subnetting problems easily.I think the concept is not clear to me as to why we consider it and the need to include it when we can do without it.Can anyone explian in a layman's language.
Thanks in advance.

Futura

There are lots of thread explaining this already.

Subnet zero is the first subnet. You used to not be able to use it and now you can. Simple as that,

Ltat42a

Here's a simple visual to help.
Years ago, the Cisco IOS did not support the use of the first subnet to be used to assign to hosts.
Kinda like this -
Network = 192.168.1.0/28
This creates 16 subnets, which look like this:
1st subnet: 192.168.10.0
2nd subnet: 192.168.1.16
3rd subnet: 192.168.1.32
4th subnet 192.168.1.48
etc
etc
etc

With subnet zero enabled, the 1st subnet is not available to assign host addresses, you must use the addresses starting with the 2nd subnet.

Technology improved, IOS changed. Now we can use the first subnet (subnet zero). New routers and new IOS's have subnet zero turned on by default.

Which brings me to this....I've seen practice questions asking to configure an interface with the first available address from the 8th subnet, and subnet zero is disabled.


hth

Somnipotent

Per Cisco the subnet zero is the first subnet obtained after subnetting a network address, for example:

Let's say you are allocated the following subnet: 192.168.1.0/27

You have "borrowed" 3 bits from the classful /24 giving you a max subnet number of 8 (2^3=8, where 3 is the number of bits borrowed) with 30 hosts per subnet (2^5=32 minus 2=30, where 5 is the remaining host bits, and minus 2 for the network and broadcast addresses).

Your 8 subnets will be:

192.168.1.0
192.168.1.32
192.168.1.64
192.168.1.96
192.168.1.128
192.168.1.160
192.168.1.192
192.168.1.224

If ip-subnet zero is disabled, the first subnet, 192.168.1.0, will be unavailable for use, thus making your available subnets 7, starting with 192.168.1.32

Apparently this was a source of confusion for some engineers back in the day and it's hard to find instances now where subnet zero is not used.