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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 Block Sizes and "Classful" Network Equivalents 1 2 3 4 IP Datagram Encapsulation and Formatting

First Level of Division

The “pie” is initially cut down the middle by using the single left-most host ID bit as an extra network bit. Let's see our network address block, 71.94.0.0/15 in binary, with the left-most host ID bit shown highlighted:

01000111 01011110 00000000 00000000

To make the split, we make one network equal to this binary network address with the highlighted bit remaining zero, and the other one with it changed to a one. This creates two subnetworks—not subnets as in the “classful” sense of the word, but portions of the original network—which I have numbered based on the numeric value of what is substituted into the new network ID bits:

Subnetwork #0: 01000111 01011110 00000000 00000000

Subnetwork #1: 01000111 01011111 00000000 00000000

Since bit #16 is now also part of the network address, these are “/16” networks, the size of a “classful” Class B network. So, the subnetworks are:

Subnetwork #0: 71.94.0.0/16

Subnetwork #1: 71.95.0.0/16

You’ll notice that the “#0” subnetwork has the same IP address as the larger network it came from; this is always true of the subnetwork 0 in a network.

IP Classless Addressing Block Sizes and "Classful" Network Equivalents 1 2 3 4 IP Datagram Encapsulation and Formatting