The following steps explain how to assign addresses with classless inter 

domain routing.

Step #1: Divide the address block 200.25.16.0/20 into two equally sized

slices. Each block represents one half of the address space, or 2,048

(211) 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 equally

sized slices. Each block represents one fourth of the address space, or

1,024 (210) 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

equally sized blocks. Each block represents one eighth of the address

space, or 512 (29) 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 

Comparing CIDR to VLSM

CIDR and VLSM both allow a portion of the IP address space to be

recursively divided into subsequently smaller pieces. The difference is

that with VLSM, the recursion is performed on the address space previ 

ously assigned to an organization and is invisible to the global Internet.

CIDR, on the other hand, permits the recursive allocation of an address

block by an Internet Registry to a high level ISP, a mid level ISP, a low 

level ISP, and a private organization's network. 

Like VLSM, the successful deployment of CIDR has three prerequisites:

  The routing protocols must carry network prefix information with

each route advertisement.

  All routers must implement a consistent forwarding algorithm based

on the longest match.

  For route aggregation to occur, addresses must be assigned so that

they are topologically significant. 

Controlling the Growth of Internet's Routing Tables

CIDR helps control the growth of the Internet's routing tables by reduc 

ing the amount of routing information. This process requires that the

Internet be divided into addressing domains. Within a domain, detailed

information is available about all of the networks that reside in the

domain. Outside of an addressing domain, only the common network

prefix is advertised. This allows a single routing table entry to specify a

route to many individual network addresses. 

U N D E R S TA N D I N G   I P   A D D R E S S I N G

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