CIDR vs Subnetting
SubNetting is a way of assigning some entity a smaller number of IP addresses than a class C. This process provides for a more efficient use of IP addresses.
CIDR (ClasslessInterDomainRouting) allows us to aggregate subnets for routing purposes. Thus if we have 8 class C networks (x.x.0.0 - x.x.7.255) assigned to a ISP, and we could route to that ISP with one routing entry x.x.0 with network length 11. If the next eight were assigned to someone else we could have a rouing entry for them of x.x.8.0 length 11 also.