Classless Internet Domain Routing (CIDR)
Well this is what I call a lifesaver…Do you remember the problem with 32-bit IPV4? Classless Internet routing is what really solved the routing problem and was originally documented in 1993 RFC 1517, 1518, 1519, and 1520.
Because of the limitation of class A, B, and C, the IETF proposed the protocol to eliminate these barriers by efficiently allocating existing IPV4 addresses, and the result is IPV6.
But how were things actually fixed? IPv6 or CIDIR is very clever, it actually take advantage of these classes A, B, or C. Any given network prefix, will actually define the amount of IP addresses.
Let's say that we have a 18.104.22.168 /20
So we have 12 bits left for our host correct? (212) = 4096 these are the actual corresponding hosts on each network, very cool isn't it? Look at the following block of networks
|Class A||10.25.40.0 /20||00001010||00011001||00000000|
|Class B||22.214.171.124 /20||10000000||00100100||00000000|
|Class C||192.168.1.0 /20||11000000||10101000||00000000|
Even though CIDR can solve a lot of problems, it may cause you a lot of problems too… be informed of what type of networks you encounter, especially ISPs. Remember IPV6 is fairly new and it is still under deployment. And yes, CIDR uses the address space very efficiently.
Here is an example, let's say that you are an ISP and you have a block of 126.96.36.199 /18
One of your clients just asked you for 1000 IP addresses. You could easily create a subnet /22 extended network prefix.
So 4 bits were required to create the extended network prefix that is (24) = 16 subnets and leaves 10 bits for the hosts. Then this is (210) = 1024 hosts per subnet; the result is that you could easily give the client one of the 16 subnets.
That's it … Congratulations! Now you know about network engineering.