Leigh Anne,
Shortly after I sent my e-mail, I found the email thread in my archive. I
also saved it because I found it to be facinating, I was quite shocked to
find it in a test exam for the CCNA in the form of a True/False. It appears
the answer may be true, but the "legal" answer is false. (Mental Note: What
is red may be black).
Someone tried it and they found it did not work on IOS (11.3 6T), or Linux.
Here is a copy of the original post, which was agreed to theoretically work,
but will not work due to RFC 1812's declariation of illegality.
Enjoy!
<Start Post from July 8th, 1999>
This is a purely theoretical question, and I see absolutely no practical
reason for wanting to do this (other than complete insanity), but I'm
curious...
Is it possible to use a subnet mask with discontiguous 1s? For example, a
subnet mask of 255.255.255.170 (expand it to binary - the last octet is
10101010)?
This could give you networks and hosts of (say)
x.x.x.128 (network A)
x.x.x.129 (host on A)
x.x.x.130 (network B)
x.x.x.131 (host on B)
x.x.x.132 (host on A)
x.x.x.133 (host on A)
x.x.x.134 (host on B)
(my binary arithmetic may be wrong but hopefully you can see what I'm
getting at)
If this is possible, are there any situations where it might be applicable
(short of a really nasty CCIE lab, maybe :-)?
<End Post from July 8th, 1999>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> Leigh Anne Chisholm
> Sent: Saturday, September 02, 2000 7:45 PM
> To: Casey Fahey; [EMAIL PROTECTED]
> Subject: RE: Subnet Question
>
>
> This was a fascinating topic that was previously discussed back
> in July of 1999. I bookmarked it for reference, because I found
> the debate so interesting. You might want to search the archives
> for the thread "Theoretical question".
>
> To summarize the jist of the discussion, RFC 1812, declares
> discontiguous subnet masks as illegal. You'll find the specifics
> in Section 10.2.2, "Address and Prefix Initialization".
>
>
> -- Leigh Anne
>
> > -----Original Message-----
> > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> > Casey Fahey
> > Sent: Saturday, September 02, 2000 7:48 PM
> > To: [EMAIL PROTECTED]
> > Subject: Re: Subnet Question
> >
> >
> > Golly, sorry to tell you this, but I think you are on the 'proverbial
> > crack'... I would answer B on any Cisco, MS, Nortel or any other
> > test you
> > care to put in front of me...
> >
> > A subnet mask determines the place where the bits on the left
> > (the network
> > bits) are divided from the bits on the right (the host bits). If
> > you had a
> > subnet mask of 0.255.255.0 you would basicly be saying that the
> > network you
> > are on is the center 2 octets only, while the host bits are the
> first and
> > last octet.
> >
> > I can't say that I have tried this on a private or a public
> > network, but I
> > cannot see this working. Either the equipment would keep you
> > from setting
> > this up, or the network would just not work. One of the central
> > assumptions
> > of TCP/IP is that the network MUST be on the right and the hosts
> > MUST be on
> > the left.
> >
> > That said, if you ever try this PLEASE let me know how it goes!
> >
> > HTH,
> >
> > Casey
> >
> > >From: "Aaron Moreau-Cook" <[EMAIL PROTECTED]>
> > >Reply-To: "Aaron Moreau-Cook" <[EMAIL PROTECTED]>
> > >To: <[EMAIL PROTECTED]>
> > >Subject: Subnet Question
> > >Date: Sat, 2 Sep 2000 17:47:43 -0700
> > >
> > >Question taken from the CCNA Exam Cram book by Walters, Rees, and Coe.
> > >
> > >A subnet mask can have a value of 0.255.255.0
> > >
> > >A) True
> > >B) False
> > >
> > >The Cisco answer would dictate that it is false, and in all
> > functionality
> > >it
> > >is true. Hypothetically though it could be true, I rememeber
> > this discusion
> > >a while ago, but I'm looking to see if I am smoking the
> proverbial crack.
> > >
> > >Thanks all!
> > >
> > >Aaron Moreau-Cook
> > >Finally taking his CCNA test this coming Friday....
> > >
> > >___________________________________
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> >
> >
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