A /24 means the first 24 bits of the address are the network prefix, leaving 8 bits for hosts. That gives 256 total addresses (2 to the power of 8), of which 254 are usable because the first address is the network address and the last is the broadcast address. The equivalent subnet mask is 255.255.255.0.

How many hosts fit in a /24?

A /24 holds 256 total IPv4 addresses. Two are reserved: the network address (the first) and the broadcast address (the last), so 254 addresses are usable for hosts. The general formula is 2 to the power of (32 minus the prefix) for total addresses, minus 2 for usable hosts in IPv4.

What is a subnet mask?

A subnet mask marks which bits of an IPv4 address are the network portion and which are the host portion. Network bits are 1 and host bits are 0, so 255.255.255.0 sets the top 24 bits as network, which is the same as writing /24 in CIDR notation. Masks and CIDR prefixes are two ways to say the same thing.

What are the private IP ranges?

RFC 1918 reserves three private IPv4 ranges that are not routed on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Home and office networks use these behind NAT. The link-local block 169.254.0.0/16 and the carrier-grade NAT block 100.64.0.0/10 are also non-public.

What is CIDR notation?

CIDR stands for Classless Inter-Domain Routing. It writes a network as an address followed by a slash and a prefix length, such as 192.168.1.0/24, where the number after the slash counts how many leading bits are the network prefix. It replaced the older fixed Class A, B, and C system and allows flexible block sizes.

What is the difference between /24 and 255.255.255.0?

They describe the same network. /24 is CIDR notation and 255.255.255.0 is the dotted-decimal subnet mask; both mark the first 24 bits as the network. CIDR is more compact and is what most modern tools and routing tables use.

Does this cheat sheet send anything to a server?

No. Every entry is baked into the page and all filtering runs in your browser with JavaScript. Open DevTools and watch the Network tab while you search: it makes zero requests.

CIDR and Subnetting Cheat Sheet

Every common CIDR block, subnet mask, reserved range, and IPv6 prefix, searchable. Filter by category and read host counts at a glance.

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CIDR prefixes, subnet masks, reserved IPv4 ranges, and IPv6 notation with host counts and meanings
Notation	Meaning
`/8` CIDR	16,777,216 total addresses (2^24), mask 255.0.0.0. Usable hosts: 16,777,214 after the network and broadcast addresses.
`/16` CIDR	65,536 total addresses (2^16), mask 255.255.0.0. Usable hosts: 65,534.
`/24` CIDR	256 total addresses, mask 255.255.255.0. Usable hosts: 254. A single common LAN.
`/25` CIDR	128 total addresses, mask 255.255.255.128. Usable hosts: 126. Splits a /24 into two.
`/26` CIDR	64 total addresses, mask 255.255.255.192. Usable hosts: 62. Splits a /24 into four.
`/27` CIDR	32 total addresses, mask 255.255.255.224. Usable hosts: 30. Splits a /24 into eight.
`/28` CIDR	16 total addresses, mask 255.255.255.240. Usable hosts: 14. Splits a /24 into sixteen.
`/29` CIDR	8 total addresses, mask 255.255.255.248. Usable hosts: 6. Handy for a small group of servers.
`/30` CIDR	4 total addresses, mask 255.255.255.252. Usable hosts: 2. The classic point-to-point link size.
`/31` CIDR	2 total addresses, mask 255.255.255.254. Both addresses are usable as endpoints on a point-to-point link (RFC 3021).
`/32` CIDR	1 address, mask 255.255.255.255. A single host route, often used for loopbacks or host ACL entries.
`255.255.255.0` mask	Equals /24. 24 network bits, 256 total addresses, 254 usable hosts.
`255.255.255.128` mask	Equals /25. 25 network bits, 128 total addresses, 126 usable hosts.
`255.255.255.192` mask	Equals /26. 26 network bits, 64 total addresses, 62 usable hosts.
`255.255.255.224` mask	Equals /27. 27 network bits, 32 total addresses, 30 usable hosts.
`255.255.255.240` mask	Equals /28. 28 network bits, 16 total addresses, 14 usable hosts.
`255.255.255.248` mask	Equals /29. 29 network bits, 8 total addresses, 6 usable hosts.
`255.255.255.252` mask	Equals /30. 30 network bits, 4 total addresses, 2 usable hosts.
`0.0.0.255 (wildcard)` mask	A wildcard mask is the bitwise inverse of a subnet mask, used by ACLs and OSPF. 0.0.0.255 is the inverse of 255.255.255.0, so it matches a /24.
`10.0.0.0/8` range	Private range (RFC 1918): 10.0.0.0 to 10.255.255.255. About 16.7 million addresses, not routed on the public internet.
`172.16.0.0/12` range	Private range (RFC 1918): 172.16.0.0 to 172.31.255.255. About 1 million addresses.
`192.168.0.0/16` range	Private range (RFC 1918): 192.168.0.0 to 192.168.255.255. 65,536 addresses, common on home routers.
`127.0.0.0/8` range	Loopback range: 127.0.0.0 to 127.255.255.255. 127.0.0.1 is the standard localhost address.
`169.254.0.0/16` range	Link-local (APIPA) range: 169.254.0.0 to 169.254.255.255. Auto-assigned when no DHCP server responds.
`100.64.0.0/10` range	Carrier-grade NAT shared range (RFC 6598): 100.64.0.0 to 100.127.255.255. Used between ISP equipment, not for public routing.
`network address` range	The first address in a subnet, with all host bits set to 0. It names the subnet itself and is not assigned to a host in IPv4.
`broadcast address` range	The last address in a subnet, with all host bits set to 1. It reaches every host on the subnet and is not assigned to a single host.
`usable host range` range	In IPv4 the assignable hosts are every address between the network and broadcast addresses, so total addresses minus 2.
`::1` IPv6	The IPv6 loopback address, equivalent to 127.0.0.1 in IPv4. Written in full as 0:0:0:0:0:0:0:1.
`fe80::/10` IPv6	Link-local range. Every IPv6 interface has one; it is valid only on the local link and is never routed.
`/64` IPv6	The standard IPv6 subnet size. 64 network bits leave 64 host bits, which SLAAC address autoconfiguration expects.
`fc00::/7` IPv6	Unique local addresses (ULA), the IPv6 counterpart to private RFC 1918 space. In practice fd00::/8 is used.
`::` IPv6	The unspecified address (all zeros). Used as a source when a host has no address yet, similar to 0.0.0.0 in IPv4.
`2000::/3` IPv6	The block currently assigned for global unicast addresses, which is the routable public IPv6 space.
`:: (shorthand)` IPv6	The double colon replaces one run of consecutive all-zero groups so an address can be written more compactly. It may appear only once per address.

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What this tool does

This is a searchable cheat sheet for IPv4 subnetting and the most common IPv6 notation. Every entry lists a notation (a CIDR prefix like /24, a dotted-decimal subnet mask, a reserved address range, or an IPv6 prefix) alongside a plain-English meaning with the total and usable host counts where they apply. Type in the filter box to narrow the table by notation or meaning, or tap a category button to show just one family. The whole dataset is baked into the page, so it works offline and sends nothing anywhere.

How to use it

Start typing in the Filter box. Entering /26 jumps to the 64-address block; entering 255.255.255.0 surfaces the /24 mask; entering private shows the RFC 1918 ranges. The category buttons (All, CIDR prefixes, Subnet masks, Address ranges, IPv6) combine with the text filter, so you can pick CIDR prefixes and type point-to-point to land on the /30 and /31 rows. Clear the box to see everything again. Nothing is submitted and there is no result limit.

Common use cases

Sizing a subnet: how many usable hosts fit in a /26, and which mask that is.
Translating between CIDR prefixes and dotted-decimal subnet masks in either direction.
Checking whether an address falls in a private, loopback, link-local, or CGNAT range.
Recalling the standard IPv6 subnet size and the common reserved prefixes.
Studying for networking interviews or certifications where subnet math comes up.

Common pitfalls

Total addresses are not usable hosts. In IPv4 you subtract 2 from the total (the network address and the broadcast address), so a /24 has 256 addresses but only 254 usable hosts.
/31 and /30 are both point-to-point sizes. A /30 gives 2 usable hosts the classic way, while a /31 (RFC 3021) uses both of its 2 addresses as endpoints with no network or broadcast address reserved.
172.16.0.0/12 is wider than it looks. The private block spans 172.16.0.0 through 172.31.255.255, not just 172.16. A /16 like 172.16.0.0/16 covers only the first slice of it.
IPv6 does not do host-minus-2 math. IPv6 subnets are almost always /64 and there is no broadcast address, so the IPv4 usable-host formula does not carry over.

Frequently asked questions

What does /24 mean?: A /24 means the first 24 bits of the address are the network prefix, leaving 8 bits for hosts. That gives 256 total addresses (2 to the power of 8), of which 254 are usable because the first address is the network address and the last is the broadcast address. The equivalent subnet mask is 255.255.255.0.
How many hosts fit in a /24?: A /24 holds 256 total IPv4 addresses. Two are reserved: the network address (the first) and the broadcast address (the last), so 254 addresses are usable for hosts. The general formula is 2 to the power of (32 minus the prefix) for total addresses, minus 2 for usable hosts in IPv4.
What is a subnet mask?: A subnet mask marks which bits of an IPv4 address are the network portion and which are the host portion. Network bits are 1 and host bits are 0, so 255.255.255.0 sets the top 24 bits as network, which is the same as writing /24 in CIDR notation. Masks and CIDR prefixes are two ways to say the same thing.
What are the private IP ranges?: RFC 1918 reserves three private IPv4 ranges that are not routed on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Home and office networks use these behind NAT. The link-local block 169.254.0.0/16 and the carrier-grade NAT block 100.64.0.0/10 are also non-public.
What is CIDR notation?: CIDR stands for Classless Inter-Domain Routing. It writes a network as an address followed by a slash and a prefix length, such as 192.168.1.0/24, where the number after the slash counts how many leading bits are the network prefix. It replaced the older fixed Class A, B, and C system and allows flexible block sizes.
What is the difference between /24 and 255.255.255.0?: They describe the same network. /24 is CIDR notation and 255.255.255.0 is the dotted-decimal subnet mask; both mark the first 24 bits as the network. CIDR is more compact and is what most modern tools and routing tables use.
Does this cheat sheet send anything to a server?: No. Every entry is baked into the page and all filtering runs in your browser with JavaScript. Open DevTools and watch the Network tab while you search: it makes zero requests.

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