[dfox]

It is other way around: in the original class-ful internet the numerical range of first octet directly implied what is in CIDR called netmask length. The original IPv4 implementations probably did not even have concept of netmask and this was instead hardcoded. Implementing the routing decision as netmask is nice optimalization which then probably inspired the CIDR concept, because at sufficently high level the only thing you need for that to work is making the netmask (or at least the length) freely configurable.