Subnet Mask & Wildcard Quick Map

The single-octet values you memorize. A mask octet of 224 means 3 bits borrowed; its wildcard is 31.

Bits	Mask octet	Mask binary	Wildcard	Wild binary	Block size
/0	0	00000000	255	11111111	256
/1	128	10000000	127	01111111	128
/2	192	11000000	63	00111111	64
/3	224	11100000	31	00011111	32
/4	240	11110000	15	00001111	16
/5	248	11111000	7	00000111	8
/6	252	11111100	3	00000011	4
/7	254	11111110	1	00000001	2
/8	255	11111111	0	00000000	1

Bits = how many of the 8 octet bits are network bits (added on top of the previous /8 boundary).

// binary place values — read any octet in your head

128

2⁷

2⁶

2⁵

2⁴

2³

2²

2¹

2⁰

Add the values left-to-right for each 1 bit. 11100000 = 128+64+32 = 224 · the first 0 marks where the wildcard starts.

CIDR	Block (2ⁿ)	Usable hosts	Formula
/30	4	2	2² − 2
/29	8	6	2³ − 2
/28	16	14	2⁴ − 2
/27	32	30	2⁵ − 2
/26	64	62	2⁶ − 2
/25	128	126	2⁷ − 2
/24	256	254	2⁸ − 2
/23	512	510	2⁹ − 2
/22	1,024	1,022	2¹⁰ − 2
/16	65,536	65,534	2¹⁶ − 2