>Group,
> In determining valid subnetworks it used to be true all were valid accept
>the all 1's subnet and the all 0's subnet. In an environment where all
>devices are compatible, I understand, that by using the configuration
>command "ip subnet-zero" it enables the use of the all 0's subnet.
The first thing to remember is that modern routers don't have a
concept of network or subnet. Classful notation and subnetting is
more a crutch for people than anything else. Routers understand
prefixes, which are no more or less than contiguous numbers of bits
starting on the left.
The following all are perfectly legitimate /20's:
10.0.0.0/20
172.16.240.0/20
192.255.240.0/20
The confusion around all ones and all zeroes subnet comes from a lack
of prefix length information, and situations in which the router
either had to infer the prefix from the high order "class" bits, or
from local configuration information. Now, modern routing protocols
transmit explicit lengths.
>The
>question I ask then, wouldn't that leave the all 1's subnet still an invalid
>subnet even when using the previous mentioned command? or is there something
>else that allows the all 1's subnetwork? I have been asked this question,
>and I don't feel I am correct. The new formula noted in the CCNA book notes
>the new equation for determining the amount of valid subnetworks is 2^# of
>subnet bits, and I figured it would be 2^#of subnet bits - 1 (for the all
>1's subnet). I don't pose this question without first a bit of research and
>everything I have read indicates the "ip subnet-zero" command only enables
>the use of the all 0's subnetwork. I however cannot find anything indicating
>the use of the all 1's subnet is permitted. Any help is appreciated.
>
> >>>Brian
My study question to you would be:
why shouldn't all 0's and all 1's work?
You may want to consult RFC 1812, and possibly 1879.
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