RE: Stupid Question maybe?

["David Edelman" <dedelman () iname com>]

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I seem to remember that before the advent of VLSM and CIDR there was no requirement for the 1 bits in the netmask to be 
contiguous with no intervening 0 bits and there was always someone who tested it out on a production network just to 
prove a point (usually only once) 

Dave

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From: NANOG <nanog-bounces () nanog org> On Behalf Of Naslund, Steve
Sent: Tuesday, December 18, 2018 3:37 PM
To: nanog () nanog org
Subject: RE: Stupid Question maybe?

It is a matter of machine readability vs human readability.  Remember the IP was around when routers did not have a lot 
of horsepower.  The dotted decimal notation was a compromise between pure binary (which the equipment used) and human 
readability.  VLSM seems obvious now but in the beginning organizing various length routes into very expensive memory 
and low horsepower processors meant that it was much easier to break routes down along byte boundaries.  This meant you 
only had four different lengths of route to deal with.  It was intended to eliminate multiple passes sorting the 
tables.  I am not quite sure what you mean about interspersing zeros, that would be meaningless.  Remember that it is a 
mask.  The address bits which are masked as 1s are significant to routing.  The bits that are masked with 0s are the 
host portion and don't matter to the network routing table.  

Steven Naslund
Chicago IL


> /24 is certainly cleaner than 255.255.255.0.
>
> I seem to remember it was Phil Karn who in the early 80's suggested that expressing subnet masks as the number of bits 
> from the top end of the address word was efficient, since subnet masks were always a series of ones followd >by zeros 
> with no interspersing, which was incorporated (or independently invented) about a decade later as CIDR a.b.c.d/n 
> notation in RFC1519.
>       - Brian
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