What is "ip subnet-zero"

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Dec 31st, 2008
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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.

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Giuseppe Larosa Wed, 12/31/2008 - 01:59
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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


elkono200 Wed, 12/31/2008 - 02:01
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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

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