LJ 56: 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