What is "ip subnet-zero"

whiteford · ‎12-31-2008

Hi,

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.

Giuseppe Larosa · ‎12-31-2008

Hello Andy,

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

Example:

192.168.5.0/24 Class C major network

If we divide it in /28 subnets

192.168.5.0/28

192.168.5.16/28

192.168.5.32/28

...

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

192.168.5.0/28

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 192.168.5.0/24 and 192.168.5.0/28 without problems

Hope to help

Giuseppe

whiteford · ‎12-31-2008

That's great, thanks.

elkono200 · ‎12-31-2008

hi,

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.

ie.

Class C subnetted - 4 networks

192.168.10.0-63

192.168.10.64-127

192.168.10.128-191

192.168.10.192-255

without "ip subnet-zero" you shouldnt use the 1st and last subnetted network, because it could be a whole Class C network so the 192.168.10.0 is the network and 192.168.10.255 is the broadcast from it.

hope that helps.

elkono

whiteford · ‎12-31-2008

Sure does thanks!