People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years. This was first proposed in 1992 as a scheme called Supernetting. Under supernetting, the classful subnet masks are extended so that a network address and subnet mask could, for example, specify multiple Class C subnets with one address. For example, If I needed about 1000 addresses, I could supernet 4 Class C networks together:
126.96.36.199 (11000000.00111100.10000000.00000000) Class C subnet address 188.8.131.52 (11000000.00111100.10000001.00000000) Class C subnet address 184.108.40.206 (11000000.00111100.10000010.00000000) Class C subnet address 220.127.116.11 (11000000.00111100.10000011.00000000) Class C subnet address -------------------------------------------------------- 18.104.22.168 (11000000.00111100.10000000.00000000) Supernetted Subnet address 255.255.252.0 (11111111.11111111.11111100.00000000) Subnet Mask 22.214.171.124 (11000000.00111100.10000011.11111111) Broadcast addressIn this example, the subnet 126.96.36.199 includes all the addresses from 188.8.131.52 to 184.108.40.206. As you can see in the binary representation of the subnet mask, the Network portion of the address is 22 bits long, and the host portion is 10 bits long.
Under CIDR, the subnet mask notation is reduced to a simplified shorthand. Instead of spelling out the bits of the subnet mask, it is simply listed as the number of 1s bits that start the mask. In the above example, instead of writing the address and subnet mask as
220.127.116.11, Subnet Mask 255.255.252.0the network address would be written simply as:
18.104.22.168/22which indicates starting address of the network, and number of 1s bits (22) in the network portion of the address. If you look at the subnet mask in binary (11111111.11111111.11111100.00000000), you can easily see how this notation works.
The use of a CIDR notated address is the same as for a Classful address. Classful addresses can easily be written in CIDR notation (Class A = /8, Class B = /16, and Class C = /24)
It is currently almost impossible for an individual or company to be allocated their own IP address blocks. You will simply be told to get them from your ISP. The reason for this is the ever-growing size of the internet routing table. Just 10 years ago, there were less than 5000 network routes in the entire Internet. Today, there are over 100,000. Using CIDR, the biggest ISPs are allocated large chunks of address space (usually with a subnet mask of /19 or even smaller); the ISP's customers (often other, smaller ISPs) are then allocated networks from the big ISP's pool. That way, all the big ISP's customers (and their customers, and so on) are accessible via 1 network route on the Internet. But I digress.
It is expected that CIDR will keep the Internet happily in IP addresses for the next few years at least. After that, IPv6, with 128 bit addresses, will be needed. Under IPv6, even sloppy address allocation would comfortably allow a billion unique IP addresses for every person on earth! The complete and gory details of CIDR are documented in RFC1519, which was released in September of 1993.