[ih] The netmask

[Andrew Walding]

Hi Michael,
Before the classful system, the first octet was the network address, and
the three remaining octets were the hosts.  That's right, in the experiment
we now call the internet, no one could see a need for more than 256
networks!

That all changed of course, but the recognition that 256 networks was not
going to be enough I think can be attributed to Internet Engineering Note
46 written by David Clark and Danny Cohen around 1976-1978.  They basically
concluded that there was going to be an exhaustion of the 256 network
boundary.

So in 1981 John Postel's famous RFC 790 introduced the classful system you
refer to.  From today's perspective it is a simple robbing of the first 4
bits of the 32 bit address to indicate what octets demarc the network
address boundary:
0 = Class A - this left 1-127 for all the class A's
1 0 = Class B - this left 128-191 for the class B's
1 1 0 = Class C - this left 192-223 for class C's
1 1 1 0 = Class D - this used 224 and up for Multicast - well not really
because Class E as you mention was also later specified, but simply
reserved, not used.

This system served the internet for 12 years or so.  Not bad, not as you
say, not efficient.

Then as you mention we went to a more efficient use of the bit boundary
with subnet and supernet masks, rather that the byte boundary that the
first four bits specified in RFC 790.

One minor thing I would like to correct is that the mask is only
communicated in routing messages, no data packets ever carry the mask.

Hope that helps.
Andy



On Tue, Jan 7, 2025 at 7:47 AM Michael Grant via Internet-history <
internet-history at elists.isoc.org> wrote:

> Before CIDR sub-netting there were fixed subnets: A, B, C, D, & E.  (and
> from memory D and E came later).  What was the rational for this being
> represented as an actual bit-mask which could have been represented as a
> number of bits like we do today?  I know that not many protocols send
> the mask over the wire, aside from perhaps routing protocols.  Did any
> early protocols use say just 5 or even just 2 bits to represent classes
> before things went to CIDR?  I never saw anything like ifconfig report
> "Class C", it was always represented as 255.255.255.0.
>
> I realize it's more efficient from a computing point of view to deal
> with bit-masks.  But I'm curious, from a historic point of view, why it
> wasn't just a number of bits or even just a number representing the
> class (A, B, C)?  In the old days when every byte of memory was sacred,
> it seems like it would have been thought of as wasteful.
> --
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> Internet-history at elists.isoc.org
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>


-- 
Best Regards,
Andy Walding
(cell: 214-405-3708)

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