Host Implications for CIDR Deployment
There may be severe host implications when CIDR based networks are
deployed. Since many hosts are classful, their user interface will not
permit them to be configured with a mask that is shorter than the nat 
ural mask for a traditional classful address.
For example, to deploy as a /20 to define a network capable
of supporting 4,094 (212  2) hosts, ensure that the software executing
on each end station will allow a traditional Class C ( to be
configured with a 20 bit mask since the natural mask for a Class C net 
work is a 24 bit mask. If the host software supports CIDR, shorter
masks can be configured. 
There will be no host problems by deploying the (a tra 
ditional Class C) allocation as a block of 16 /24s since non CIDR hosts
will interpret their local /24 as a Class C. Likewise, (a tra 
ditional Class B) could be deployed as a block of 255 /24s since the hosts
will interpret the /24s as subnets of a /16. If host software supports the
configuration of shorter than expected masks, the network manager has
tremendous flexibility in network design and address allocation. 
Efficient Address Allocation
How does CIDR lead to the efficient allocation of the IPv4 address
space? In a classful environment, an Internet Service Provider (ISP) can
only allocate /8, /16, or /24 addresses. In a CIDR environment, the ISP
can carve out a block of its registered address space that specifically
meets the needs of each client, provides additional room for growth,
and does not waste a scarce resource. 
Assume that an ISP has been assigned the address block
This block represents 16,384 (214) IP addresses, which can be inter 
preted as 64 /24s. If a client requires 800 host addresses, rather than
assigning a Class B address (and wasting approximately 64,700
addresses) or four individual Class C addresses (and introducing four
new routes into the global Internet routing tables), the ISP could assign
the client the address block, which is a block of 1,024
(210) IP addresses (four contiguous /24s). The efficiency of this alloca 
tion is illustrated in Figure 29. 
F I G U R E   2 9 .   C I D R   E ff i c i e n t   A d d re s s   A l l o c a t i o n
U N D E R S TA N D I N G   I P   A D D R E S S I N G
3 4






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