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CIDR Address Allocation Example


CIDR Address Allocation Example

For this example, assume that an ISP owns the address block 200.25.0.0/16. This block represents 65, 536 (2 16 ) IP addresses (or 256 /24s).

From the 200.25.0.0/16 block it wants to allocate the 200.25.16.0/20 address block . This smaller block represents 4,096 (2 12 ) IP addresses (or 16 /24s).

Address Block 11001000.00011001.00010000.00000000 200.25.16.0/20
In a classful environment, the ISP is forced to use the /20 as 16 individual /24s.

ipadf29.gif (3k)

Figure 29: Slicing the Pie - Classful Environment

However, in a classless environment, the ISP is free to cut up the pie any way it wants. It could slice up the original pie into 2 pieces (each 1/2 of the address space) and assign one portion to Organization A, then cut the other half into 2 pieces (each 1/4 of the address space) and assign one piece to Organization B, and finally slice the remaining fourth into 2 pieces (each 1/8 of the address space) and assign it to Organization C and Organization D. Each of the individual organizations is free to allocate the address space within its "Intranetwork" as it sees fit. This is illustrated in Figure 30.

ipadf30.gif (1k)

Figure 30: Slicing the Pie - Classless Environment

Step #1: Divide the address block 200.25.16.0/20 into two equal size slices. Each block represents one-half of the address space or 2,048 (2 11 ) IP addresses.

ISP's Block 11001000.00011001.00010000.00000000 200.25.16.0/20
Org A: 11001000.00011001.00010000.00000000 200.25.16.0/21
Reserved: 11001000.00011001.00011000.00000000 200.25.24.0/21

Step #2: Divide the reserved block (200.25.24.0/21) into two equal size slices. Each block represents one-fourth of
the address space or 1,024 (2 10 ) IP addresses.

Reserved 11001000.00011001.00011000.00000000 200.25.24.0/21
Org B: 11001000.00011001.00011000.00000000 200.25.24.0/22
Reserved 11001000.00011001.00011100.00000000 200.25.28.0/22

Step #3: Divide the reserved address block (200.25.28.0/22) into two equal size blocks. Each block represents one-eight of the address space or 512 (2 9 ) IP addresses.

Reserved 11001000.00011001.00011100.00000000 200.25.28.0/22
Org C: 11001000.00011001.00011100.00000000 200.25.28.0/23
Org D: 11001000.00011001.00011110.00000000 200.25.30.0/23

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