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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

Third Level of Division

We now take each of the four /18 networks above and further subdivide it. We want to make each of these contain a number of blocks of different sizes corresponding to our potential customers. One way to do this would be as follows:

• Larger Organizations: Customers needing up to 510 addresses require a /23 network. We divide sub-subnetwork #1-0, 71.95.0.0/18 by taking five bits from the host ID field:

01000111 01011111 00000000 00000000

We substitute into these five bits 00000, 00001, 00010 and so on, giving us 32 different /23 networks in this block, each containing 9 bits for the host ID, for 510 hosts. The first will be sub-sub-subnetwork #1-0-0, 71.95.0.0/23; the second sub-sub-subnetwork #1-0-1, 71.95.2.0/23; the last will be sub-sub-subnetwork #1-0-31: 71.95.62.0/23.

• Medium-Sized Organizations: For customers needing up to 254 addresses, we divide sub-subnetwork #1-1, 71.95.64.0/18, by taking six bits from the host ID field:

01000111 01011111 01000000 00000000

This gives us 64 different /24 networks. The first will be sub-sub-subnetwork #1-1-0, 71.95.64.0/24, the second sub-sub-subnetwork #1-1-1, 71.95.65.0/24, and so on.

• Smaller Organizations: For customers with up to 126 hosts, we divide sub-subnetwork #1-2, 71.95.128.0/18, by taking seven bits from the host ID field:

01000111 01011111 10000000 00000000

Seven bits allow 128 of these /25 networks within our /18 block. The first will be 71.95.128.0/25, the second 71.95.128.128/25, the third 71.95.129.0/25, and so on.

• Very Small Organizations: For customers with up to 60 hosts, we divide sub-subnetwork #1-3, 71.95.192.0/18, by taking eight bits from the host ID field:

01000111 01011111 11000000 00000000

This gives us 256 different /26 networks within our /18 block. The first will be 71.95.192.0/26, the second 71.95.192.64/26, and so on.

Above all else, CIDR is about flexibility—this is only one of many different ways to slice up this pie (sheet of brownies, whatever!) The ISP might decide that creating four different sizes of customer networks in advance was not the right way to go. They might instead just take the tack of dividing the pie in half, dividing it in half again, and so on, as many times as needed to create “pie slices” of the right size. Alternately, if most of their customers need around 50, 100, 200 or 500 hosts, the example above might be the easiest to administer.

It would still be possible for the ISP to further divide any of the smaller blocks further if they needed. They could split a /26 sub-sub-subnetwork into four /28 sub-sub-sub-subnetworks for very small customers, for example. Also, an individual customer of this ISP could do the same thing, dividing their own block to suit the internal structure of their network.

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