what's the ' ip zero-subnet ' command for ?

Hi all

what's the ' ip zero-subnet ' command for ?

Thanks.

 

:what's the ' ip zero-subnet ' command for ?

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.

 

for the 0 and 255 subnet...all zero's or all 1's

ie:
10.0.0.0 255.255.255.0
or
10.0.255.0 255.255.255.0

 

Thanks.
It's a very detail explanation.

 

It is now.

http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
says:
"ip subnet-zero
To enable the use of subnet 0 for interface addresses and routing
updates, use the ip subnet-zero command in global configuration mode.
To restore the default, use the no form of this command.

ip subnet-zero

no ip subnet-zero

Syntax Description
This command has no arguments or keywords.

Defaults
Enabled"

It has I think been the default for a while.

 

:> > considered to be wrong. "ip subnet-zero" is not the default,

:It is now.

:http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
:says:
:"ip subnet-zero
:To enable the use of subnet 0 for interface addresses and routing
:updates, use the ip subnet-zero command in global configuration mode.
:To restore the default, use the no form of this command.

But if is on by default, and "no ip subnet-zero" restores the default,
then the implication is you cannot turn it off. I don't think
that is supposed to be the case, so the documentation you pointed
to is likely wrong.

 

On 25 Oct 2004 15:51:15 GMT, -cnrc.gc.ca (Walter Roberson) wrote:

~ In article <>,
~
~ :> > considered to be wrong. "ip subnet-zero" is not the default,
~
~ :It is now.
~
~ :http://www.cisco.com/univercd/cc/td.../ios123/123cgcr/ipras_r/ip1_i2g.htm#wp1081869
~ :says:
~ :"ip subnet-zero
~ :To enable the use of subnet 0 for interface addresses and routing
~ :updates, use the ip subnet-zero command in global configuration mode.
~ :To restore the default, use the no form of this command.
~
~ But if is on by default, and "no ip subnet-zero" restores the default,
~ then the implication is you cannot turn it off. I don't think
~ that is supposed to be the case, so the documentation you pointed
~ to is likely wrong.

It looks like that documentation is indeed wrong, I'm afraid.

It states there that, by default, "ip subnet-zero" is enabled.
However, I checked, and as of 12.3(11)T, "ip subnet-zero" is
still the default.

The fact that both "ip subnet-zero" and "no ip subnet-zero" are
visible in the configuration does imply that we may be plotting
to change the default in a future release.

What IS correct is that "no ip subnet-zero" restores the default
- although what would be MORE correct would be to state that
the command "default ip subnet-zero" restores the default ... as
it is the case that, in general, the "default" command can be counted
on to restore any command to its default value, regardless of whether
it is enabled or disabled by default.

I'll put this on my list of stuff to get fixed.

Aaron

 

I didn't read the whole entry, I was satisfied with:-

"Defaults Enabled"

which matched with my existing understanding
and my own view of the meaning of 'default' which is
something like "unconfigured default" and did not consider
'IOS config file default' which is the condition where no
config entry is displayed. I had never even heard of the IOS
config command "default ... ...".


Anyway:-


(C837-K9O3SY6-M), Version 12.3(4)T3, RELEASE SOFTWARE (fc3)

wr erase
reload

.. . . . .

Router#sh run | inc subnet
ip subnet-zero


Router#conf t
Router(config)#no ip subnet-zero
Router#sh run | inc subnet
no ip subnet-zero


Router#conf t
Router(config)#ip subnet-zero
Router#sh run | inc subnet
ip subnet-zero


Router#conf t
Router(config)#default ip subnet-zero
Router#
sh run | inc subnet
no ip subnet-zero
Router#


Router#conf t
Router(config)#default ip subnet-zero
Router#sh run | inc subnet
no ip subnet-zero
Router#


It seems that the "default command" does not return the
"ip subnet-zero" state to the "router unconfigured" state
which is what I would expect.

(fc4) coming soon?<g>

 

~ > ~ :> > considered to be wrong. "ip subnet-zero" is not the default,
~ > ~
~ > ~ :It is now.
~ > I'll put this on my list of stuff to get fixed.
~ >
~ > Aaron

( where I assert that "no ip subnet-zero" is still the default
as of 12.3(11)T, [although I typoed it and said that "ip subnet-zero"
is the default], so our documentation that "ip subnet-zero"
is the default is wrong ... )

~ I didn't read the whole entry, I was satisfied with:-
~
~ "Defaults Enabled"
~
~ which matched with my existing understanding
~ and my own view of the meaning of 'default' which is
~ something like "unconfigured default" and did not consider
~ 'IOS config file default' which is the condition where no
~ config entry is displayed. I had never even heard of the IOS
~ config command "default ... ...".
~
~
~ Anyway:-
~
~
~ (C837-K9O3SY6-M), Version 12.3(4)T3, RELEASE SOFTWARE (fc3)
~
~ wr erase
~ reload
~
~ . . . . .
~
~ Router#sh run | inc subnet
~ ip subnet-zero
~
~
~ Router#conf t
~ Router(config)#no ip subnet-zero
~ Router#sh run | inc subnet
~ no ip subnet-zero
~
~
~ Router#conf t
~ Router(config)#ip subnet-zero
~ Router#sh run | inc subnet
~ ip subnet-zero
~
~
~ Router#conf t
~ Router(config)#default ip subnet-zero
~ Router#
~ sh run | inc subnet
~ no ip subnet-zero
~ Router#
~
~
~ Router#conf t
~ Router(config)#default ip subnet-zero
~ Router#sh run | inc subnet
~ no ip subnet-zero
~ Router#
~
~
~ It seems that the "default command" does not return the
~ "ip subnet-zero" state to the "router unconfigured" state
~ which is what I would expect.

Now I'm REALLY confused

~ (fc4) coming soon?<g>

It had better not be! The fc<n> value is only bumped up while
the IOS version is in internal testing ... e.g. 12.3(4)T3 (fc3)
means that while we were testing 12.3(4)T3 (fc1) we found a showstopper
bug and so built (fc2) then found another showstopper bug and then
built (fc3) which is what you are running now (unless you got some
code that escaped from our lab.) Once an IOS release is posted to
CCO, the fc<n> value is never incremented; a rebuild from this
branch would appear as 12.3(4)T4 (fc<n>).

Cheers,

Aaron

 

assign last subnet addresses while using no ip subnet-zero command

hi,

I use these command in IOS 1.x and 2.x. but when i use no ip subnet-zero command i found i m not able to use first subnet addresses but i can able to assign last subnet addresses.
How It is possible....