>The "because we say so" analogy seems basically accurate. In the real
>world I, and I suspect many of you, have used all zeros and ones
>successfully. However, when taking certainly the ccna exam, and possibly
>others, the attitude to take is,"Cisco says zeros won't work without ip
>subnet zero(which is true), and ones shouldnt be used."
A slight clarification of the Cisco position: without subnet zero,
you can't _enter_ subnet zero. As long as the routing protocols are
classless and receive subnet zero (or the all ones), everything will
work just fine.
This position, of course, is simply that of the training and
documentation folk. Any ISP engineer would laugh their head off.
There is, of course, the real world and the Cisco world. If you have
registered address space, ask for more, and you consistently haven't
used the all zeroes and all ones subnets, and the registry audits
you, your address request will probably bounce.
In the general Internet, use of the all-zeroes and all-ones subnets
are standard. The registries' policy is that if your router can't
handle them, fix the router.
>
>Brian "Sonic" Whalen
>Success = Preparation + Opportunity
>
>
>On Sun, 15 Apr 2001, David Chandler wrote:
>
>> Chuck
>>
>> Thanks for the proof read :>
>>
>> Bellow is the cisco page & part of the doc relating to zero subnets.
>> To me it reads "don't; because we say so"
>>
>>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios121/121cgcr/ip_c/ipcprt1/1cdipadr.htm#xtocid105602
>>
>> -----------------------
>> Enabling Use of Subnet Zero
>>
>> Subnetting with a subnet address of zero is illegal and strongly
>discouraged
>> (as
>> stated in RFC 791) because of the confusion that can arise between a
>network
>> and
>> a
>> subnet that have the same addresses. For example, if network 131.108.0.0
is
>> subnetted as 255.255.255.0, subnet zero would be written as
>> 131.108.0.0which is
>>
>> identical to the network address.
>>
>> You can use the all zeros and all ones subnet (131.108.255.0), even
though
>> it is
>> discouraged. Configuring interfaces for the all ones subnet is explicitly
>> allowed.
>> However, if you need the entire subnet space for your IP address, use the
>> following command in global configuration mode to enable subnet zero:
>> ---------------------------
>>
>> You mentioned that Windows is not rfc1812 compiant and that it allows
wacky
>> subnets and disallows some valid subnets. Was that trial & error or has
>> microsoft documented this? I hate spending an hour looking for a document
>> that
>> is not there...
>>
>> Thanks
>>
>> DaveC
>>
>>
>> Chuck Larrieu wrote:
>>
>> > Comments within:
>> >
>> > -----Original Message-----
>> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf
Of
>> > David Chandler
>> > Sent: Saturday, April 14, 2001 11:25 PM
>> > To: [EMAIL PROTECTED]
>> > Subject: designing subnets with all ones/zeros.. [7:695]
>> >
>> > I have two questions regarding using the all ones and/or the all zeros
>> > subnet.
>> >
>> > Recently one of my co-workers started studying for CCNA and while
>> > reviewing subnets he kept telling me that you could not use the all
zero
>> > or all ones subnet.
>> >
>> > CL: classically speaking this is true. Early implementations, etc.
these
>> > days this is no longer the case
>> >
>> > The Win95, NT, and LINUX hosts didn't have a
>> > problem with it nor did the routers.
>> >
>> > CL: a long time ago on this list we had a discussion of wacky subnet
>masks.
>> > In the course of researching this, I found that the windows IP stack
was
>> not
>> > rfc 1812 compliant in that it allowed discontiguous / wacky / non
>> contiguous
>> > ones subnet masks, and that windows also categorically denied use of
>> certain
>> > legitimate ip addresses. Such as 172.16.1.255/16 I believe that this
is
>> > corrected in Win2K
>> >
>> > I tested it with RIP & EIGRP.
>> > (skipped OSPF since it is classful).
>> >
>> > CL: I believe you meant to say "classless" ;->
>> >
>> > I found that Cisco and others vendors agree that it will work, but they
> > > "Strongly discourage using the all ones or all zeros subnets"
>> >
>> > CL: where did you find language about "strongly discourage"?
> > >
>> > PS: if some of you try testing this; note that prior to 12.1 you'll
need
>> > to enter
>> > (config)# ip zero-subnet
>> > before the router will accept a zero subnet on a interface. Starting in
>> > 12.1 the zero subnet is enabled by default.
>> >
>> > CL: ip subnet-zero
>> >
>> > Question #1: What type problems could you run into by using a all
>> > ones/zero subnet.
>> >
>> > CL: issues with older equipment / obsolete equipment / old OS versions
>> >
>> > Question #2: For you folks that are in design; Do you follow or
>> > ignore the "DO NOT USE ALL ONES/ZEROS" rule?
>> >
>> > CL: use both all the time. Of course I sell new Cisco equipment, so
there
>> is
>> > no issue with most customers. Or I sell EIGRP or OSPF designs. Same
>thing.
>> > ;->
>> >
>> > I'm trying to get a real world idea of what the standard practice is.
>> > I work at a large corp, so I haven't a clue what sane people do.
>> >
>> > CL: so do I and neither do I.
>> >
> > > DaveC
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=1505&t=695
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