CIDR / Subnet Mask Reference Table

A reference table of subnet masks, wildcard masks, and usable host counts for every prefix length from /0 to /32. Enter how many hosts you need and the smallest CIDR block that fits is highlighted automatically.

Last updated: 2026-09-10

CIDR reference table (/0 to /32)

The subnet mask, wildcard mask, total address count, and usable host count for every prefix length.

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

Usable hosts excludes the network and broadcast addresses (except /31, which RFC 3021 allows to use both of its 2 addresses, and /32, counted as a single address).

What the CIDR and subnet mask table is

In CIDR notation, “/24” means that the first 24 bits of an IPv4 address are fixed as the network portion. This table sets out **all thirty-three lengths from /0 to /32** with the subnet mask, the wildcard mask, the total address count and the usable host count for each. The total is two raised to the power of thirty-two less the prefix length — 256 addresses for a /24.

**The usable host count is the total less two**, because the network address at the front and the broadcast address at the end cannot be assigned to equipment. There are two exceptions. **A /31 may have both of its addresses assigned, under RFC 3021**, which serves to save addresses on a point-to-point link between routers. **A /32 counts as one**, a single address — a host route. Enter the number of devices you need to accommodate and the smallest block that holds them is picked out as a row.

How to use the table

Enter the number of devices Give the count you need to accommodate and the tool identifies **the smallest block that holds them** — the one with the longest prefix.
Read the row it names At the prefix length identified, read off the subnet mask you will actually configure.
Keep the two reserved addresses in mind A /24 suffices for 254 devices, but 255 calls for a /23, since the network and broadcast addresses cannot be used.
Take the wildcard mask from the table An ACL on Cisco equipment, or an OSPF network statement, wants the inverted mask. Copy it straight from the row.
Remember the two exceptional rows A /31 gives 2 and a /32 gives 1. They fall outside the general rule of total less two and are handled separately.

Tips for getting more out of it

"/24" means 255.255.255.0 (256 addresses, 254 usable hosts) — the most common size for a small office LAN or home network.
/31 is a special case where both addresses are usable (RFC 3021). It's commonly used on point-to-point router links to save address space.
If you don't know the exact number of hosts you need, enter your headcount or device count in the box above and the smallest CIDR block that fits will be highlighted in the table below.
A wildcard mask — the inverse of a subnet mask — is used in Cisco ACL (access control list) rules and OSPF network statements.

Where the table helps

Designing an office LAN

The staff and device counts settle the block you need. A /24 is the commonest choice for a small office or a home network.

Splitting a network into subnets

Dividing by department, work out the smallest block for each and add them up to get the total you require.

Configuring a link between routers

A point-to-point link needs only a /30 (two usable) or a /31 (two, under RFC 3021). Whether you can use a /31 depends on the equipment.

Carving up a cloud VPC

Providers reserve differing numbers of addresses in each subnet, so what you can actually use may come out below the usable count here.

When you want to check addresses themselves

For validity of form, see the IP address checker; for your own address, what is my IP; for subnet arithmetic, the subnet calculator.

Terms about CIDR

CIDR: Classless Inter-Domain Routing — the notation that states a range by the number of bits in its network portion, as in “/24”.
Prefix length: How many bits from the front of the address form the network portion. The longer it runs, the smaller the block.
Subnet mask: A 32-bit value with ones for the network portion and zeros for the host portion. For a /24 it comes to 255.255.255.0.
Wildcard mask: The subnet mask with its bits inverted. It is what ACLs on Cisco equipment and OSPF configuration call for.
Network address: The first address of a block, standing for the network itself. It is not assigned to equipment.
Broadcast address: The last address of a block, used to send to every device on that network at once. It is not assigned to equipment.
RFC 3021: The specification permitting both addresses of a /31 to be assigned to the two ends of a point-to-point link. It saves addresses.
Host route: A route naming a single address, as a /32 does. It serves to state a route to one particular machine.

FAQ

A /24 contains 256 IP addresses (0–255), but the first address (network) and the last (broadcast) usually can't be assigned to a device, leaving 254 addresses actually usable by hosts.

A subnet mask marks the network portion with 1s and the host portion with 0s (e.g. 255.255.255.0); a wildcard mask is its inverse (e.g. 0.0.0.255). Cisco router access lists and OSPF configuration use wildcard masks, so it helps to know how the two correspond.

RFC 3021 formally allows a /31 to use both of its addresses on point-to-point links such as router-to-router connections. A /32 refers to a single IP address as a host route, used in routing tables to direct traffic to one specific host.

Enter the number of hosts you need in the box at the top of this tool, and it automatically determines the smallest CIDR block (prefix length) that fits, highlighting the matching row in the table below. If you expect future growth, it's common practice to pick a slightly larger block than you strictly need today.

Side Note — IPv4 address exhaustion and the birth of CIDR

CIDR (Classless Inter-Domain Routing) was introduced in 1993. Before that, networks could only be sized as Class A (/8), Class B (/16), or Class C (/24) — three fixed sizes. Assigning a single Class B reserved 65,534 host addresses, so handing one to an organization that only needed a few thousand wasted vast numbers of addresses; in the early 1990s, this waste was accelerating IPv4 address exhaustion significantly.

CIDR solved this by letting the prefix length be chosen freely, so a block could be sized much closer to the actual number of hosts needed. This dramatically improved address-space efficiency and extended IPv4's practical lifespan well beyond what had been expected. CIDR also made it possible to summarize multiple contiguous networks into a single route — "route aggregation" — which helped keep the internet's global routing tables from growing out of control.

New public IPv4 allocations have largely run out today, and the industry keeps moving toward IPv6, but CIDR notation is still used daily to design subnets inside corporate LANs and cloud VPCs. Deciding "do I need a block a bit bigger than a /24?" remains a routine, practical skill for cloud engineers.

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