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 200.25.16.0 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 (200.25.16.0) 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 200.25.16.0/20 (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, 130.14.0.0/16 (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 206.0.64.0/18.

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 206.0.68.0/22, 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