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New Member

What is "ip subnet-zero"


I seem to be stuggling with the ip subnet-zero. It came up in an exam when it said pc1 needs to contact pc2 (with a few routers in between) and asked how many zero subnets did it pass through to get to pc2, I got it wrong and a few more questions came up about it. It's hardly mentioned in my Cisco ICND1 and 2 book.

Hall of Fame Super Silver

Re: What is "ip subnet-zero"

Hello Andy,

when IP subnetting was introduced at first the first block was reserved to avoid confusion with the network subnet.

Example: Class C major network

If we divide it in /28 subnets


the ip subnet-zero that is now a default allows to use the first

the old rule for the subnets you can get was

if K=Subnet.len - network.len

2^K -2

so in our example would be 2^4-2 14

now you can use also the first subnet and the last one.

Because modern routing protocols are classless they carry the mask of updates they can handle and without problems

Hope to help


New Member

Re: What is "ip subnet-zero"

That's great, thanks.

New Member

Re: What is "ip subnet-zero"


ip subnet-zero (default active on new ciscos) is a command that you allow to use the first and last subnet on a subnetted Class C network.


Class C subnetted - 4 networks

without "ip subnet-zero" you shouldnt use the 1st and last subnetted network, because it could be a whole Class C network so the is the network and is the broadcast from it.

hope that helps.


New Member

Re: What is "ip subnet-zero"

Sure does thanks!