Subnet Mask Cheat Sheet

A complete IPv4 CIDR table covering all 25 usable prefixes (/8 through /32), with subnet mask, usable host count, wildcard mask (for ACLs, OSPF, EIGRP) and typical deployment notes. For interactive subnet math, open the Subnet Calculator.

CIDR	Subnet Mask	Addresses	Usable Hosts	Wildcard Mask	Common Use
/8	255.0.0.0	16,777,216	16,777,214	0.255.255.255	Class-A block, RIR allocation
/9	255.128.0.0	8,388,608	8,388,606	0.127.255.255	Very large ISP space
/10	255.192.0.0	4,194,304	4,194,302	0.63.255.255	CGNAT (`100.64.0.0/10`)
/11	255.224.0.0	2,097,152	2,097,150	0.31.255.255	Large ISP pool
/12	255.240.0.0	1,048,576	1,048,574	0.15.255.255	RFC 1918 private `172.16.0.0/12`
/13	255.248.0.0	524,288	524,286	0.7.255.255	Mid-size ISP pool
/14	255.252.0.0	262,144	262,142	0.3.255.255	Carrier sub-block
/15	255.254.0.0	131,072	131,070	0.1.255.255	Large campus
/16	255.255.0.0	65,536	65,534	0.0.255.255	Class-B block, `192.168.0.0/16` private
/17	255.255.128.0	32,768	32,766	0.0.127.255	Large campus / DC zone
/18	255.255.192.0	16,384	16,382	0.0.63.255	Data-center pod
/19	255.255.224.0	8,192	8,190	0.0.31.255	Large enterprise
/20	255.255.240.0	4,096	4,094	0.0.15.255	Site / branch
/21	255.255.248.0	2,048	2,046	0.0.7.255	Large department
/22	255.255.252.0	1,024	1,022	0.0.3.255	Standard branch
/23	255.255.254.0	512	510	0.0.1.255	Standard office
/24	255.255.255.0	256	254	0.0.0.255	Class-C, typical LAN
/25	255.255.255.128	128	126	0.0.0.127	LAN segment
/26	255.255.255.192	64	62	0.0.0.63	Small VLAN
/27	255.255.255.224	32	30	0.0.0.31	Printer / IoT zone
/28	255.255.255.240	16	14	0.0.0.15	Small DMZ
/29	255.255.255.248	8	6	0.0.0.7	Point-to-point + 4 hosts
/30	255.255.255.252	4	2	0.0.0.3	Classic point-to-point
/31	255.255.255.254	2	2	0.0.0.1	P2P per RFC 3021
/32	255.255.255.255	1	1	0.0.0.0	Single host, loopback, ACL

Usable hosts vs. total addresses

Each subnet typically reserves two addresses: the network address (all host bits 0) and the broadcast address (all host bits 1). Usable hosts = 2^n - 2 where n is the number of host bits.

Exceptions:

/31 (RFC 3021) — used for point-to-point links. Both addresses are usable; no broadcast.
/32 — a single host route. Common for loopbacks, ACL match-lines, or specific routes.

Private and reserved ranges

Per RFC 1918 / RFC 6598:

Range	Size	Purpose
`10.0.0.0/8`	16.7M addresses	RFC 1918 — largest private block
`172.16.0.0/12`	1.05M addresses	RFC 1918 — mid-size private block
`192.168.0.0/16`	65,536	RFC 1918 — home/SOHO networks
`100.64.0.0/10`	4.2M	RFC 6598 — Carrier-Grade NAT
`169.254.0.0/16`	65,536	Link-local (APIPA)
`127.0.0.0/8`	16.7M	Loopback (`127.0.0.1`)

Wildcard mask in 60 seconds

The wildcard mask is the bitwise inverse of the subnet mask. NOT 255.255.255.0 = 0.0.0.255. It is used by Cisco ACLs and the OSPF/EIGRP network statement. A 0 bit means must match; a 1 bit means don't care.

Practical examples

Home LAN: typically 192.168.1.0/24 (254 usable hosts)
VPN pool: often 10.8.0.0/24 or 10.10.0.0/16
Docker default: 172.17.0.0/16
Kubernetes pod CIDR: commonly 10.244.0.0/16
AWS VPC: recommended /16 base, subnets carved as /24

Calculate a specific network with the Subnet Calculator.
Set up reverse-DNS zones for /24 blocks? See DNS Record Types Reference.
Foundational read: Subnet Calculation step-by-step.
Glossary: CIDR notation, Subnet, Wildcard mask.