be a former Class A, Class B, or Class C address. Routers that support
CIDR do not make assumptions based on the first three bits of the
address, they rely on the prefix length information provided with the
In a classless environment, prefixes are viewed as bitwise contiguous
blocks of the IP address space. For example, all prefixes with a /20 pre 
fix represent the same amount of address space (212 or 4,096 host
addresses). Furthermore, a /20 prefix can be assigned to a traditional
Class A, Class B, or Class C network number. Figure 28 shows how each
of the following /20 blocks represent 4,096 host addresses,, and 
F I G U R E   2 8 .   B i t w i s e   C o n t i g u o u s   A d d re s s   B l o c k s
Table 3 provides information about the most commonly deployed CIDR
address blocks. The table shows that a /15 allocation can also be speci 
fied using the traditional dotted decimal mask notation of
Also, a /15 allocation contains a bitwise contiguous block of 128K
(131,072) IP addresses that can be classfully interpreted as two Class B
networks or 512 Class C networks. 
TA B L E   3 .   C I D R   A d d re s s   B l o c k s
3 3






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