Hello Rick
You got most of it, but I hope this clarifies it a bit more.
CIDR is Classless Inter Domain Routing. This feature allows routing to occur not only between classful addresses but also between classless addresses. Classless addresses are those that allow a variable length subnet mask (VLSM), those that have a subnet mask other than the allowed classful address. So VLSM is the feature of IP addresses that allows CIDR to take place.
When subnetting, you always start with a larger address space and subnet into smaller address spaces. The large address space may be classful or it may be classless. For example, an ISP may provide you with a range of addresses of 147.52.0.0/22 and you have to subnet them appropriately into your network. This range is a class B address, but its subnet mask is not class B. So when you subnet, you are usually given the address space that is available to you and you are to subnet it into subnets that suit your needs.
Here you have to understand the difference between the subnet mask of the classful addresses, and the prefix being used to describe the full range of private addresses.
In the case of 10.0.0.0/8, there is one possible class A private address using the classful subnet mask of /8. Both the subnet mask and the prefix defining the full range of private addresses is the same.
In the second case, the full range of private addresses is 172.16.0.0 -172.31.255.255 which can be defined as 172.16.0.0/12. But the subnet mask of class B addresses is /16. So:
172.16.0.0/12 says that these are ALL of the private IP addresses that can be used, this essentially defines the range.
/16 says that the classful addresses of this range must have a subnet mask of /16. So the following are some classful class B private address subnets:
- 172.16.0.0/16
- 172.17.0.0/16
- 172.18.0.0/16
- 172.19.0.0/16
all of which fall within the available range of private addresses.
For the third case, the full range of class C private IP addresses that can be used are defined by 192.168.0.0/16 which is the same as saying 192.168.0.0 - 192.168.255.255. But because these are class C addresses, they must have a /24 subnet, so if you are using classful networks, then some possible private class C addresses are:
- 192.168.10.0/24
- 192.168.11.0/24
- 192.168.12.0/24
- 192.168.13.0/24
So 8/ 12/ 16/ are the prefixes that define the full range of private addresses while /8 /16 /24 are the classful subnet masks of the subnets for each type.
I hope this has been helpful!
Laz