What is the subnet mask for /24, /16 and /8?

/24 is 255.255.255.0 with 254 usable hosts, /16 is 255.255.0.0 with 65,534, and /8 is 255.0.0.0 with 16,777,214. These are also the three classful sizes the Classful filter shows.

How many usable hosts are in a /26 or a /27?

A /26 has 64 addresses and 62 usable hosts; a /27 has 32 addresses and 30 usable hosts. Usable is the total minus the network and broadcast addresses.

Why does a /31 have 2 usable hosts when the formula gives 0?

RFC 3021 says that on a point-to-point link with a 31-bit mask the two addresses must be treated as host addresses, with no network or broadcast address. A /30 spends 4 addresses on two hosts, a /31 spends 2.

What subnet mask do I need for 26 hosts?

A /27, 255.255.255.224: it has 32 addresses and 30 usable hosts, while a /28 holds only 14. Pick the longest prefix whose usable count is at least what you need; the Hosts needed field does this for any number, and the table under it lists the break points from 2 hosts (/31) to 65,534 (/16).

How many hosts does a /32 have?

A /32 is a single address, and the tool shows 1 usable host. That is this tool’s convention: RFC 3021 does not mention /32.

How do I find the prefix for a mask like 255.255.254.0?

Type the mask in the search box. 255.255.254.0 is /23, with 512 addresses and 510 usable hosts. The wildcard 0.0.1.255 is the mask with every bit flipped.

Subnet Mask Cheat Sheet

Subnet mask table for every IPv4 prefix from /0 to /32: dotted mask, wildcard, total and usable hosts, with /31 counted as 2 (RFC 3021) and /32 as 1. How it works

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

Which mask fits my hosts?

Rule: pick the smallest block (the longest prefix) whose usable hosts are at least the number you need.

Hosts needed

Hosts needed, with the smallest mask that holds them
Hosts needed (up to)	Host Bits	CIDR	Subnet Mask
2	1	/31	255.255.255.254
6	3	/29	255.255.255.248
14	4	/28	255.255.255.240
30	5	/27	255.255.255.224
62	6	/26	255.255.255.192
126	7	/25	255.255.255.128
254	8	/24	255.255.255.0
510	9	/23	255.255.254.0
1,022	10	/22	255.255.252.0
2,046	11	/21	255.255.248.0
4,094	12	/20	255.255.240.0
8,190	13	/19	255.255.224.0
16,382	14	/18	255.255.192.0
32,766	15	/17	255.255.128.0
65,534	16	/16	255.255.0.0

A /31 (2 hosts) has no network or broadcast address (RFC 3021). Equipment that reserves those two addresses needs a /30 for 2 hosts.

Usable hosts are the total minus the network and broadcast addresses, except /31 (2: both addresses are hosts on a point-to-point link, RFC 3021) and /32 (1: a single host address).

Showing 33 of 33 entries — Values computed from the prefix length for a 32-bit address space

About the Subnet Mask Cheat Sheet

A /24 is the mask 255.255.255.0, with the wildcard 0.0.0.255, 256 addresses and 254 usable hosts. The table covers every IPv4 prefix from /0 to /32 with its dotted mask, wildcard, total addresses and usable hosts. A /31 counts 2 usable hosts (RFC 3021) and a /32 counts 1, because the usual total minus 2 does not fit them. Type a host count under Which mask fits my hosts? and it names the smallest mask: 26 hosts need /27 (255.255.255.224, 30 usable).

The eight mask octet values and the prefixes that end in them

Mask octet	1 bits in the octet	Wildcard octet	Prefixes
128	1	127	/1, /9, /17, /25
192	2	63	/2, /10, /18, /26
224	3	31	/3, /11, /19, /27
240	4	15	/4, /12, /20, /28
248	5	7	/5, /13, /21, /29
252	6	3	/6, /14, /22, /30
254	7	1	/7, /15, /23, /31
255	8	0	/8, /16, /24, /32

A valid mask is a run of 255s, then one of these values, then 0s. 192 in the fourth octet is /26 (255.255.255.192), in the third /18 (255.255.192.0). Each wildcard octet is 255 minus the mask octet.

Hosts needed and the smallest mask that holds them

Hosts needed (up to)	Host bits	CIDR	Subnet mask
2	1	/31	255.255.255.254
6	3	/29	255.255.255.248
14	4	/28	255.255.255.240
30	5	/27	255.255.255.224
62	6	/26	255.255.255.192
126	7	/25	255.255.255.128
254	8	/24	255.255.255.0
510	9	/23	255.255.254.0
1,022	10	/22	255.255.252.0
2,046	11	/21	255.255.248.0
4,094	12	/20	255.255.240.0
8,190	13	/19	255.255.224.0
16,382	14	/18	255.255.192.0
32,766	15	/17	255.255.128.0
65,534	16	/16	255.255.0.0

The rule the tool uses: the longest prefix whose usable hosts are at least the number needed, so 26 hosts need a /27 (30 usable). 2 hosts get a /31 under RFC 3021; equipment that reserves a network and a broadcast address needs a /30 for them.

How to use this tool

Choose All, Common or Classful, or leave All showing.
Type a prefix, a mask, a wildcard or a host count in the search box. Commas and spaces are ignored, so 65536 and 65,536 both find /16.
Press Copy on a row to copy its prefix, mask, wildcard, total and usable hosts as tab-separated text, or Copy table for every row shown, with a header line. Print gives a paper copy without the buttons.

Frequently asked questions

What is the subnet mask for /24, /16 and /8?: /24 is 255.255.255.0 with 254 usable hosts, /16 is 255.255.0.0 with 65,534, and /8 is 255.0.0.0 with 16,777,214. These are also the three classful sizes the Classful filter shows.
How many usable hosts are in a /26 or a /27?: A /26 has 64 addresses and 62 usable hosts; a /27 has 32 addresses and 30 usable hosts. Usable is the total minus the network and broadcast addresses.
Why does a /31 have 2 usable hosts when the formula gives 0?: RFC 3021 says that on a point-to-point link with a 31-bit mask the two addresses must be treated as host addresses, with no network or broadcast address. A /30 spends 4 addresses on two hosts, a /31 spends 2.
What subnet mask do I need for 26 hosts?: A /27, 255.255.255.224: it has 32 addresses and 30 usable hosts, while a /28 holds only 14. Pick the longest prefix whose usable count is at least what you need; the Hosts needed field does this for any number, and the table under it lists the break points from 2 hosts (/31) to 65,534 (/16).
How many hosts does a /32 have?: A /32 is a single address, and the tool shows 1 usable host. That is this tool’s convention: RFC 3021 does not mention /32.
How do I find the prefix for a mask like 255.255.254.0?: Type the mask in the search box. 255.255.254.0 is /23, with 512 addresses and 510 usable hosts. The wildcard 0.0.1.255 is the mask with every bit flipped.

Looking up a prefix

Find the mask for a prefix, or the prefix for a mask, without working the binary out. The table holds all 33 IPv4 prefixes, /0 to /32, with the dotted mask, the wildcard (inverse) mask, the total address count and the usable host count. Filter to Common (/8, /16 and /20 to /30) or Classful (/8, /16 and /24), or search by prefix, mask, wildcard or host count: 65536 and 65,536 both find /16. Each row copies as tab-separated text, with totals as plain digits; Copy table copies every row shown, and Print drops the buttons. To size a new subnet, type a host count under Which mask fits my hosts?: 26 hosts need /27. The Network Bits column is the prefix length again, the count of 1 bits in the mask, and is left out of the copied row.

How each row is computed

mask = the top n bits set to 1, wildcard = the inverse of the mask, total = 2^(32 − n), usable = total − 2. The table is computed from the prefix, not typed in.

Example: /26 has 26 mask bits, so the last octet of the mask is 192 (255.255.255.192), the wildcard is 0.0.0.63, the total is 2^6 = 64 and the usable count is 62.

Two prefixes break the subtraction. A /31 has 2 addresses and shows 2 usable, because RFC 3021 says both addresses of a 31-bit prefix on a point-to-point link are host addresses. A /32 is one address and shows 1: that is this tool's convention, not a statement in RFC 3021, which does not mention /32. The table covers IPv4 only, and it counts addresses by arithmetic: it does not know which addresses your network reserves for its own use.

Fun Fact: The Classful filter shows the sizes the internet used before RFC 1519 brought in CIDR in September 1993. Class B (/16) space was running out so fast that the RFC projected every class B network would be handed out within about 15 months.

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