The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Addressing                           9  IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages 1 2 3 4 IP Classless Addressing Block Sizes and "Classful" Network Equivalents

Common Features of "Classful" and Classless Addressing

Note that there are a few aspects of addressing that were defined under the “classful” scheme that don't change under CIDR:

• Private Address Blocks: Certain blocks of addresses are still reserved for private network addressing. These addresses are not directly routed on the Internet, but can be used in conjunction with Network Address Translation (NAT) to allow IP hosts without public addresses to access the Internet.
  
  
• Addresses With Special Meanings: The special meanings assigned to certain network ID and host ID patterns are the same as before. This is also why we still must subtract two from the number of hosts in each network: for the all-zeroes case that refers to the network as a whole, and the all-ones address used for broadcast.
  
  
• Loopback Addresses: The network 127.0.0.0 is still reserved for loopback functionality. (In CIDR it is given the notation 127.0.0.0/8).

Finally, note that use of classless addressing requires hardware and software designed to handle it. If the hardware and software are still assuming that they are operating in a “classful” environment, they will not properly interpret addresses. Since CIDR has now been around for over a decade, this is usually not a problem with modern systems.

IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages 1 2 3 4 IP Classless Addressing Block Sizes and "Classful" Network Equivalents