[NTLUG:Discuss] IP super/sub-netting maddness

[Wayne Walker]

On Wed, Sep 20, 2006 at 11:04:15AM -0700, joseph beasley wrote:
> Don't mean to be pushy....   but here goes...
> 
> Odd and invalid.  Here are a few links.
> 
> http://www.freesoft.org/CIE/Course/Subnet/6.htm
says 255.255.255.128 is valid

> http://www.pku.edu.cn/academic/research/computer-center/tc/html/TC0306.html
says 255.255.255.128 is never valid.
says you can't have 25 bit or 32 bit networks.  You can.  32
bit networks are often used by ip aliases (in BSD I believe) and by VPN
and Point to Point connections.  I have two racks at ISPs where I'm
allocated a 25 bit subnet.

Poor choice for verification of anything.

Neither of them is correct across the board.  Kenneth is right.  Odd,
recommended for Non-use, yes.  Invalid, NO.  Usable in most OS's IP
stacks, NO.  Most IP stacks will consider such a subnet mask as invalid,
but that is because that is almost never how a network is defined.

> http://www.exabyte.net/lambert/subnet/subnet_masking_summary.htm
"Are any subnet masking limitations due to Microsoft's O/S's?  No, it's
inherent in the architecture of DNS."

subnet masks and DNS have NOTHING in common.  again a site that someone
threw together to be helpful, but he doesn't know what he's talking
about.

> http://freespace.virgin.net/glynn.etherington/subnet_masks_and_ip_for_beginners.htm
"Beginning IP for New Users"

true.  far from defining the limits of what can and can't be used in a
netmask.

NOW, if you can find something supporting either side of the discussion
HERE then it's meaningful:

http://www.ietf.org/rfc/rfc0791.txt

http://tools.ietf.org/html/rfc950 shows a mask of 255.255.255.88

Wayne

> --- Kenneth Loafman <kenneth at loafman.com> wrote:
> 
> > Just a nit, but it is odd, not invalid...
> > 
> > 255.255.255.200 == FFFFFFC8 or 1..11001000
> > 
> > which means you have 5 bits to play with, just not an adjacent 5
> > bits,
> > thus there are 32 possible IPs in the subnet, C8-CF, D8-DF, E8-EF,
> > and
> > F8-FF.  This fits the def of a submask, but would not be compatible
> > with
> > CIDR notation except as 4 distinct small subnets of 8 each.
> > 
> > I've seen this used for device subnets where redundant devices are
> > subnetted.  Not spiffy, but valid.
> > 
> > ...Ken

Wayne Walker

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