There is something I've always been mixed up about with IP addressing. When a company purchases internet access from an ISP and obtains their inside global IP address that they'll use for NAT (lets say they're just using PAT and only in need of 1 inside global address to make things easy), do your private IP address schemes have to stick within the class of network you purchased from your ISP? When purchasing internet access from your ISP, does it coincide with network classes?
I'm mixed up about the process itself, considering my public IP at home for instance is 24.x.x.x (which traditionally is a class A network 0-127 range), but at home obviously we all have class C networks, so I'm confused about the whole thing.
I notice most companies use a 172.16-31.0.0 private address scheme and subnet further from there. Which is a class B private address scheme, but is this because they purchased a class B network? Or as a result of CIDR, there are no classes of networks to be purchased, the internet access is the only thing needing purchasing which you will then be given a public IP from your ISP, and the private scheme is left up to the company and Network Engineers to determine. Could a company purchase internet access from their ISP for instance, then make the internal network 10.0.0.0 and subnet from there giving themselves as many subnets and host IPs they could ever use?
If you're totally confused by my questions by now, just to sum up, I want to know basically whether or not the public IP a company is administered from their ISP when they purchase internet access (inside global IP) has any influence on your private IP address scheme (for instance, having to use 172.16.0.0/16 as a starting point instead of being able to use anything I want because of your inside global address), and then subnetting from there. Any clarity on this would be greatly appreciated. I apologize for the enormous length of this post! Thanks everyone.