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 (216) IP addresses (or 256

/24s). 

The ISP wants to allocate the smaller 200.25.16.0/20 address block,

which represents 4,096 (212) 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 individ 

ual /24s. 

F I G U R E   3 0 .   S l i c i n g   t h e   P i e   C l a s s f u l   E n v i o r n m e n t

However, in a classless environment, the ISP is free to cut up the pie

any way it wants. It could slice the original pie into  pieces (each one 

half of the address space) and assign one portion to Organization A,

then cut the other half into two pieces (each one fourth of the address

space) and assign one piece to Organization B, and then slice the

remaining fourth into two pieces (each one eighth of the address space)

and assign them to Organization C and Organization D. Each of the orga 

nizations is free to allocate the address space within its  Intranetwork 

as desired. This example is illustrated in Figure 31. 

F I G U R E   3 1 .   S l i c i n g   t h e   P i e   C l a s s l e s s   E n v i o r n m e n t

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