[ih] The netmask

[Andrew G. Malis]

Michael,

RFC 1519 goes into a lot of detail regarding the history of IP addressing
and the design decisions that led to CIDR. While the RFC refers to
bit-masks, operationally people realized that since CIDR bit-masks were
always all ones from the left and all zeros from the right, they could be
equivalently represented by an integer count of one bits, usually following
a "/" character. So then we ended up with tables like
https://docs.netgate.com/pfsense/en/latest/network/cidr.html .

Cheers
Andy



On Tue, Jan 7, 2025 at 8:47 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.
> --
> Internet-history mailing list
> Internet-history at elists.isoc.org
> https://elists.isoc.org/mailman/listinfo/internet-history
>