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

route. 

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 

10.23.64.0/20, 130.5.0.0/20, and 200.7.128.0/20. 

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 255.254.0.0.

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