CIDR: A Prescription for Shortness of Address Space

Table 3. CIDR Translations

A chart for CIDR translations:

mask  A  B  C  #  networks
.0 /8 /16   /24   1  0
.128  /9 /17   /25   2  0-128
.192  /10   /18   /26   4  0-64-128-192
.224  /11   /19   /27   8  0-32-64-96-128-160-192-224
.240  /12   /20   /28   16 0-16-32-48-64-80-96-112-128-144-160-\
               176-192-208-224-240
.248  /13   /21   /29   32 0-8-16-24-32-40-48-56-64-72-80-88-\
               96-104-112-120-128-136-144-152-\
               152-160-168-176-184-192-299-208-\
               216-224-232-240-248
.252  /14   /22   /30   64 0-4-8-12-16-20-24-28-32-36-40-44-\
               48-52-56-60-64-72-76-80-84-88-92-\
               96-100-104-108-112-116-120-124-128-\
               132-136-140-144-148-152-156-160-164-\
               168-172-176-180-184-188-192-196-200-\
               204-208-216-220-224-228-232-236-240-\
               244-248-252
.254  /15   /23   N/A   128
.255  N/A   N/A   /32

The “mask” number refers to the first number

!= 255

A, B and C refer to the position of the “mask”,

A = second octet
B = third octet
C = final octet

# = number of networks difference from classful address;
networks = network address to be used in 2d, 3d, or 4th octet. To get a networks broadcast address, go to next higher network number and subtract 1, i.e.,

netmask   = 255.240.0.0
network   = 10.16.0.0 (10.16/12)
broadcast = 10.31.255.255

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