CIDR and Netmask Table

CIDR Subnet Table

Subnet Mask	CIDR Prefix	Total IP's	Usable IP's	Number of Class C Networks
255.255.255.255	/32	1	1	1/256th
255.255.255.254	/31	2	0	1/128th
255.255.255.252	/30	4	2	1/64th
255.255.255.248	/29	8	6	1/32nd
255.255.255.240	/28	16	14	1/16th
255.255.255.224	/27	32	30	1/8th
255.255.255.192	/26	64	62	1/4th
255.255.255.128	/25	128	126	1 half
255.255.255.0	/24	256	254	1
255.255.254.0	/23	512	510	2
255.255.252.0	/22	1024	1022	4
255.255.248.0	/21	2048	2046	8
255.255.240.0	/20	4096	4094	16
255.255.224.0	/19	8192	8190	32
255.255.192.0	/18	16384	16382	64
255.255.128.0	/17	32768	32766	128
255.255.0.0	/16	65536	65534	256
255.254.0.0	/15	131072	131070	512
255.252.0.0	/14	262144	262142	1024
255.248.0.0	/13	524288	524286	2048
255.240.0.0	/12	1048576	1048574	4096
255.224.0.0	/11	2097152	2097150	8192
255.192.0.0	/10	4194304	4194302	16384
255.128.0.0	/9	8388608	8388606	32768
255.0.0.0	/8	16777216	16777214	65536
254.0.0.0	/7	33554432	33554430	131072
252.0.0.0	/6	67108864	67108862	262144
248.0.0.0	/5	134217728	134217726	1048576
240.0.0.0	/4	268435456	268435454	2097152
224.0.0.0	/3	536870912	536870910	4194304
192.0.0.0	/2	1073741824	1073741822	8388608
128.0.0.0	/1	2147483648	2147483646	16777216
0.0.0.0	/0	4294967296	4294967294	33554432

The CIDR number comes from the number of 1’s in the subnet mask when converted to binary system.

The common subnet mask 255.255.255.0 is 11111111.11111111.11111111.00000000 in binary. This adds up to 24 1’s or /24 (pronounced ‘slash twenty four’).

A subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 in binary, or 26 1’s, hence a /26 and so on….