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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

Second Level of Division

Let's say we set aside subnetwork #0 above for future ISP allocations. We then choose to divide the second subnetwork, into four. These we will then further subdivide into different sizes to meet our customer needs. To divide into four groups we need two more bits from the host ID of subnetwork #1, shown highlighted and underlined next to the original subnet bit:

01000111 01011111 00000000 00000000

These two bits are replaced by the patterns 00, 01, 10 and 11 to get four sub-subnetworks. They will be “/18” networks of course, since we took two extra bits from the host ID of a “/16”:

Sub-subnetwork #1-0: 01000111 01011111 00000000 00000000 (71.95.0.0/18)

Sub-subnetwork #1-1: 01000111 01011111 01000000 00000000 (71.95.64.0/18)

Sub-subnetwork #1-2: 01000111 01011111 10000000 00000000 (71.95.128.0/18)

Sub-subnetwork #1-3: 01000111 01011111 11000000 00000000 (71.95.192.0/18)

Each of these has 16,382 addresses.

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