CIDR house rules: IP network classes

IP addresses are often grouped (or summarized) into distinct blocks. These typically have a single administrative authority. But in the past, these were rigidly assigned in different classes. It wasn't flexible, so we moved to something called Classless Inter-Domain Routing, or CIDR.
Today, I'm talking about the history of the network classes, including how they are determined, their original classful network masks, the introduction of NAT and "private" IP space, and then CIDR and subnets!
EXTRA CREDIT:
- Network Address Translation (NAT) can be used for more than just your usual RFC1918 "private IP" space! Other examples include 100.64.0.0/10 (used for Carrier-Grade NAT, or CGN), or any time two organizations want to communicate directly but share some internal IP addressing.
- CGN is similar in application to the standard NAT we all know, but interorganizational NAT is a more complicated beast, as you might need to translate "source", "destination", or both--depending on which direction the traffic is going!
- Understanding CIDR can be a time-saver for communication. It's much faster to say or write "192.168.1.0/24" instead of "192.168.1.0 255.255.255.0". But equipment will usually want the subnet mask in dotted-decimal notation, since that's been around longer... and not everybody knows binary like a network admin!
- Subnetting is one of the hardest concepts to really learn and understand for new network admins, so you should really take your time and practice it as much as you can! A /27 subnet mask isn't common, but if you look at that and immediately know "255.255.255.224" and "30 usable IPs", then you're probably doing pretty good!

[@darrinito]

Thank you. IP address space has always confused me.