[NTLUG:Discuss] IP super/sub-netting maddness
Kenneth Loafman
kenneth at loafman.com
Wed Sep 20 13:45:03 CDT 2006
Kenneth Loafman wrote:
>>From an authority, not training papers...
>
> Douglas Comer, Internetworking With TCP/IP, 4Th ed, Vol 1, pg 156
>
> He does say that its recommended that sites use contiguous subnet masks,
> but not required. The example on that page is quite similar to the one
> we were discussing.
>
> That said, it was just a nit to point out that contiguous bits are not
> required by the standards. I remember non-contiguous bits being used
> for subnets at Convex and at Sabre. At Convex we even had regression
> tests to make sure they worked, but I can't think of a really good
> reason to use them right now.
>
> 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
>>
>> http://www.pku.edu.cn/academic/research/computer-center/tc/html/TC0306.html
>>
>> http://www.exabyte.net/lambert/subnet/subnet_masking_summary.htm
>>
>> http://freespace.virgin.net/glynn.etherington/subnet_masks_and_ip_for_beginners.htm
>>
>> --- 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
>>>
>>> joseph beasley wrote:
>>>> 255.255.255.200 is not a valid subnet mask. 255.255.255.192 or
>>>> 255.255.255.224. Just follow the bits.
>>>>
>>>> --- Wayne Walker <wwalker at bybent.com> wrote:
>>>>
>>>>> In theory, you could use 255.255.255.200 and 192.168.1.64.
>>>>>
>>>>> BUT, why do you need to refer to those machines as a subnet? For
>>>>> firewall rules?
>>>>>
>>>>> If so, just treat it as two 26 bit subnets.
>>>>>
>>>>> 192.168.1.64/26 255.255.255.192
>>>>> 192.168.1.128/26 255.255.255.192
>>>>>
>>>>> Wayne
>>>>>
>>>>> On Tue, Sep 19, 2006 at 02:18:06PM -0500, Richard Geoffrion wrote:
>>>>>> Ok... I think I'm asking the impossible here because no matter
>>> how
>>>>> I
>>>>>> calculate it the bits don't line up...but here goes.
>>>>>>
>>>>>> Can you supernet multiple subnetted network ranges when they
>>> don't
>>>>> fall
>>>>>> on bit boundaries??
>>>>>>
>>>>>> For instance,
>>>>>>
>>>>>> Networks 192.168.0.0 mask 255.255.255.0 and 192.168.1.0 mask
>>>>>> 255.255.255.0 can be referred to / supernetted by changing the
>>>>> mask to
>>>>>> 255.255.254.0. This gives 510 available hosts on the same
>>> network
>>>>>> instead of just 254.
>>>>>>
>>>>>>
>>>>>> Now... what about a situation where I have a 125 ip address DHCP
>>>>> scope
>>>>>> on a 192.168.1.0/24 segment...and (in my infinite wisdom and
>>>>> foresight)
>>>>>> I started it at 60. Yes... 60. Just pick a number...ANY number.
>>>>> Throw
>>>>>> a dart -- WHOO HOO! Triple 20! The DHCP scope is
>>>>> 192.168.1.60-185.
>>>>>> Now that the DHCP scope is set.. let's put servers and other
>>>>> special
>>>>>> statics from 1-47 and printers from 224-254... YEAH!! Let's just
>>>>> trash
>>>>>> both /25 subnets with 'stuff' we can't move.
>>>>>>
>>>>>> Now that we have all our static hosts and printers integrated...
>>>>> .how
>>>>>> about we firm up these ranges (which is where I got those numbers
>>>>> above)
>>>>>> and see if there is a way to reference the DHCP scope by network.
>>>
>>>>>> Hmmm... nope.. BUT.. if I modify the dhcp scope to
>>>>>> be...192.168.1.64-192....then the DHCP scope will fall on the
>>>>> ranges of
>>>>>> several subnets.
>>>>>>
>>>>>> So...is it possible to supernet the subnets? (I know it sounds
>>>>>> ridiculous...and unlikely...but if it's possible it would be a
>>>>> great
>>>>>> learning experience for me...not to mention making firewall rules
>>>>>> easier. :) )
>>>>>>
>>>>>> Afterthought... In the history of TCP/IP...has a subnet mask
>>> such
>>>>>> as........well...no...that'd be an invalid subnet mask. hm...
>>>>> still...
>>>>>> has something like (11111111.11111111.11111111.1101xxxx)
>>>>> 255.255.255.200
>>>>>> ever been used?
>>>>>>
>>>>>> --
>>>>>> Richard
>>>>>> Nope...I'm not smoking anything....
>>>>>> (not to say I'm not crazy...just not smok'n)
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> http://www.ntlug.org/mailman/listinfo/discuss
>>>>> --
>>>>>
>>>>> Wayne Walker
>>>>>
>>>>> www.unwiredbuyer.com - when you just can't be by the computer
>>>>>
>>>>> wwalker at bybent.com Do you use Linux?!
>>>>> http://www.bybent.com Get Counted!
>>>>> http://counter.li.org/
>>>>> Perl - http://www.perl.org/ Perl User Groups -
>>>>> http://www.pm.org/
>>>>> Jabber: wwalker at jabber.gnumber.com AIM: lwwalkerbybent
>>>>> IRC: wwalker on freenode.net
>>>>>
>>>>> _______________________________________________
>>>>> http://www.ntlug.org/mailman/listinfo/discuss
>>>>>
>>>> Joe Beasley
>>>> CNE, CCNP, MCSE, CCNA, AEIOU....
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>>>>
>>>>
>>>>
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>>>
>>
>> Joe Beasley
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>>
>>
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