rule: a routing interface cannot send routing information out the same interface upon which
it was received
Routing information can be condensed (summarized) from a longer prefix into a shorter prefix; the routing table is more efficient, reduced, and routing updates are fewer.
Classful routing protocols do not include vlsm in their updates. they are delineated by classful masking.
ex. 10.0.0.0/8 172.16.0.0/16 192.168.1.0/24 (rfc 1918)
255.0.0.0 class A 255.255.0.0 class B 255.255.255.0 class C
The ip classless command was introduced into default behavior for ios 12.0. Classful routing protocols, ie. Rip and IGRP discard traffic bound for any unknown subnet of a major network, even with a default route configured. IP classless allows for the use of a default route in this case.