The following steps explain how to assign addresses with classless inter 
domain routing.
Step #1: Divide the address block 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
Org A: 11001000.00011001.00010000.00000000
Reserved: 11001000.00011001.00011000.00000000 
Step #2: Divide the reserved block ( 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
Org B: 11001000.00011001.00011000.00000000
Reserved 11001000.00011001.00011100.00000000 
Step #3: Divide the reserved address block ( into two
equally sized blocks. Each block represents one eighth of the address
space, or 512 (29) IP addresses. 
Reserved 11001000.00011001.00011100.00000000
Org C: 11001000.00011001.00011100.00000000
Org D: 11001000.00011001.00011110.00000000 
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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