Class C Addressing

Question

I recently saw someone use '200.1.2.0/18.' This doesn't make any sense to me. This is a Class C address with a subnet mask of /18. I thought the subnet mask for a Class C had to be at least /24. How exactly does this work?

Answer 1

That address uses what's called Classless Inter-Domain Routing (or CIDR for short). The class system has largely been abolished due to the rapidly diminishing IPv4 space.