What is a CIDR cheat sheet?

A CIDR cheat sheet is a quick-reference table listing every IPv4 prefix length from /0 to /32 alongside its subnet mask, wildcard mask, total number of IP addresses, and usable host count. Network engineers use it to quickly determine the size and boundaries of a subnet without manual calculation.

What are the most common CIDR prefix lengths?

The most common CIDR prefix lengths are /24 (256 addresses, 254 usable hosts — standard LAN), /16 (65,536 addresses — campus or large network), /30 (4 addresses, 2 usable hosts — point-to-point links), and /32 (single host address). Cloud environments also frequently use /28 and /27 for small subnets.

What is the difference between CIDR notation and subnet masks?

CIDR notation and subnet masks convey the same information in different formats. CIDR notation uses a slash followed by the number of network bits (e.g., /24), while a subnet mask expresses the same value as a dotted-decimal address (e.g., 255.255.255.0). CIDR notation is more compact and is the modern standard for representing network prefixes.

CIDR Cheat Sheet

This table lists every IPv4 CIDR prefix from /0 to /32 with the corresponding subnet mask, wildcard mask, address count, and usable host count. Click any prefix to see detailed examples and use cases. Working with IPv6? See the IPv6 cheat sheet.

CIDR	Subnet Mask	Wildcard Mask	Total IPs	Usable Hosts	Network Bits	Host Bits
/32	255.255.255.255	0.0.0.0	1	1	32	0
/31	255.255.255.254	0.0.0.1	2	2	31	1
/30	255.255.255.252	0.0.0.3	4	2	30	2
/29	255.255.255.248	0.0.0.7	8	6	29	3
/28	255.255.255.240	0.0.0.15	16	14	28	4
/27	255.255.255.224	0.0.0.31	32	30	27	5
/26	255.255.255.192	0.0.0.63	64	62	26	6
/25	255.255.255.128	0.0.0.127	128	126	25	7
/24	255.255.255.0	0.0.0.255	256	254	24	8
/23	255.255.254.0	0.0.1.255	512	510	23	9
/22	255.255.252.0	0.0.3.255	1,024	1,022	22	10
/21	255.255.248.0	0.0.7.255	2,048	2,046	21	11
/20	255.255.240.0	0.0.15.255	4,096	4,094	20	12
/19	255.255.224.0	0.0.31.255	8,192	8,190	19	13
/18	255.255.192.0	0.0.63.255	16,384	16,382	18	14
/17	255.255.128.0	0.0.127.255	32,768	32,766	17	15
/16	255.255.0.0	0.0.255.255	65,536	65,534	16	16
/15	255.254.0.0	0.1.255.255	131,072	131,070	15	17
/14	255.252.0.0	0.3.255.255	262,144	262,142	14	18
/13	255.248.0.0	0.7.255.255	524,288	524,286	13	19
/12	255.240.0.0	0.15.255.255	1,048,576	1,048,574	12	20
/11	255.224.0.0	0.31.255.255	2,097,152	2,097,150	11	21
/10	255.192.0.0	0.63.255.255	4,194,304	4,194,302	10	22
/9	255.128.0.0	0.127.255.255	8,388,608	8,388,606	9	23
/8	255.0.0.0	0.255.255.255	16,777,216	16,777,214	8	24
/7	254.0.0.0	1.255.255.255	33,554,432	33,554,430	7	25
/6	252.0.0.0	3.255.255.255	67,108,864	67,108,862	6	26
/5	248.0.0.0	7.255.255.255	134,217,728	134,217,726	5	27
/4	240.0.0.0	15.255.255.255	268,435,456	268,435,454	4	28
/3	224.0.0.0	31.255.255.255	536,870,912	536,870,910	3	29
/2	192.0.0.0	63.255.255.255	1,073,741,824	1,073,741,822	2	30
/1	128.0.0.0	127.255.255.255	2,147,483,648	2,147,483,646	1	31
/0	0.0.0.0	255.255.255.255	4,294,967,296	4,294,967,294	0	32

Usable hosts excludes the network and broadcast addresses, except for /31 (point-to-point, RFC 3021) and /32 (single host).

Understanding CIDR Notation

CIDR (Classless Inter-Domain Routing) notation represents an IP address and its associated network prefix. The number after the slash indicates how many of the 32 bits in an IPv4 address are dedicated to the network portion. A smaller prefix length like /8 means fewer network bits and more host bits, resulting in a larger network with more available addresses. A larger prefix like /28 allocates more bits to the network and fewer to hosts, creating a smaller subnet. This system replaced the older classful addressing scheme (Class A, B, C) and allows for far more flexible and efficient allocation of IP address space.

Subnet Mask Chart by Network Size

The same 33 prefixes grouped by the job they do, for picking a size rather than looking one up.

/8 – /15

255.0.0.0 – 255.254.0.0

ISP and large enterprise allocations. Rarely a single broadcast domain.

/16 – /23

255.255.0.0 – 255.255.254.0

Campus and site-wide ranges, usually subdivided further before use.

/24

255.255.255.0

The standard LAN. 254 usable hosts, and the default assumption in most documentation.

/25 – /29

255.255.255.128 – 255.255.255.248

VLANs, cloud subnets and small offices. 126 down to 6 usable hosts.

/30 – /32

255.255.255.252 – 255.255.255.255

Point-to-point links, loopbacks and host routes.

/0 – /7

0.0.0.0 – 254.0.0.0

Default routes and global aggregates seen in BGP, not assigned to hosts.

Wildcard Masks

The wildcard mask column above is the bitwise inverse of the subnet mask, and it is the form Cisco access lists, OSPF and EIGRP expect. For the inverse masks on their own, with the conversion rule and worked ACL examples, see the wildcard mask cheat sheet.

Common Subnet Sizes

/24 — 254 usable hosts. The standard subnet for most LANs and small office networks.
/16 — 65,534 usable hosts. Common for campus networks and large enterprise environments.
/30 — 2 usable hosts. Ideal for point-to-point links between routers.
/32 — Single host address. Used for loopback interfaces and host routes.

Private vs Public Subnets

RFC 1918 defines three IPv4 address ranges reserved for private use. These addresses are not routable on the public internet and can be freely used within internal networks:

10.0.0.0/8 — 16,777,216 addresses (Class A private range)
172.16.0.0/12 — 1,048,576 addresses (Class B private range)
192.168.0.0/16 — 65,536 addresses (Class C private range)

Any address outside these ranges is considered public and must be assigned by a Regional Internet Registry (RIR). Most home and corporate networks use private addresses internally along with NAT (Network Address Translation) for internet access.

Try the Subnet Calculator →