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TOPIC: ip subnet zero

ip subnet zero 10 years 4 months ago #15888

  • bigal
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Hi everyone,

I'm studying for my CCNA, and there is 1 basic concept that I don't grasp. I understand everything with subnetting, however I do not understand what the "ip subnet-zero" command does.

Can someone illustrate this more clearly for me, maybe with an example of what the difference would be with and without this command on a router.

Thanks
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Re: ip subnet zero 10 years 4 months ago #15893

  • eylli
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In the days before CIDR, if you had a set of subnets induced upon a
base network address, then the first and last of those subnets
were reserved. ip subnet-zero tells the router that those subnets
are not reserved.

For example, if you were using subnets of a class C address
(say, 192.168.12.0) induced by the subnet mask 255.255.255.192
(that's 64 addresses at a chunk), then you would have four
subnets generated, 192.168.12.0-63, 192.168.12.64-127,
192.168.12.128-191, and 192.168.12.192-255 . However, before CIDR
you would not be allowed to use the first (192.168.12.0-63)
or last (192.168.12.192-255), leaving you with only two
usable subnets (192.168.12.64-127 and 192.168.12.128-191).


I've heard that this is something that the Cisco certification
exams love to trip people up on -- ask how many subnets there are
of a certain address of a certain size, and anyone who doesn't
reduce the "obvious" count by 2 because of the unusable ones is
considered to be wrong. "ip subnet-zero" is not the default,
and the cert exams like to push the edge of what the behaviour
would be under any defaults that they do not specifically state
have been changed.
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Re: ip subnet zero 10 years 4 months ago #15900

  • bigal
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thanks eylli,

its clear now! :D
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Re: ip subnet zero 10 years 4 months ago #15934

  • Tarun
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In the days before CIDR, if you had a set of subnets induced upon a
base network address, then the first and last of those subnets
were reserved. ip subnet-zero tells the router that those subnets
are not reserved.

For example, if you were using subnets of a class C address
(say, 192.168.12.0) induced by the subnet mask 255.255.255.192
(that's 64 addresses at a chunk), then you would have four
subnets generated, 192.168.12.0-63, 192.168.12.64-127,
192.168.12.128-191, and 192.168.12.192-255 . However, before CIDR
you would not be allowed to use the first (192.168.12.0-63)
or last (192.168.12.192-255), leaving you with only two
usable subnets (192.168.12.64-127 and 192.168.12.128-191).


I've heard that this is something that the Cisco certification
exams love to trip people up on -- ask how many subnets there are
of a certain address of a certain size, and anyone who doesn't
reduce the "obvious" count by 2 because of the unusable ones is
considered to be wrong. "ip subnet-zero" is not the default,
and the cert exams like to push the edge of what the behaviour
would be under any defaults that they do not specifically state
have been changed.

Nice explanation...!!
Next would be SP (Service Provider)
CCNA, CCNP (Switching), CCIE#20640
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Re: ip subnet zero 10 years 4 months ago #16006

  • jonty509
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goooddd onee...
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