Every device on a network has an address like 192.168.1.10 and a mask like 255.255.255.0, and together they decide which other addresses it can reach directly. This subnet calculator turns that pair into the block it describes: where the block starts and ends, and how many devices fit in it.
Type the address and pick the prefix, the number after the slash, or the mask beside it. It works out the network and broadcast addresses, the first and last address a device can use, the mask and the wildcard mask, and every step is shown. Each figure it supplies names the standard it comes from, and the list of what it cannot tell you is below the answer.
In this guide
The subnet calculator
The number before the first dot: 192 in 192.168.1.10.
The number after the first dot: 168 in 192.168.1.10.
The number after the second dot: 1 in 192.168.1.10.
The last number: 10 in 192.168.1.10. Any address in the network gives the same answer.
The number after the slash, as in 192.168.1.10/24. If you have a mask instead, pick the line that shows it: 255.255.255.0 is /24.
How that is worked out, line by line
- The address you typed192.168.1.10
- Addresses in the block256
- First usable address192.168.1.1
- Last usable address192.168.1.254
- Wildcard mask0.0.0.255
On those numbers you are in the Room for one home network band, which is the section below.
This runs entirely inside your browser. Nothing you type is sent anywhere, saved or read by this site, and the page keeps working with the network unplugged.
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Where every number here comes from
No part of this was measured on a bench here. This site works from published specifications and from what owners report, and every number below is either one you typed in or a published figure with its source named next to it.
The figures this page supplies
- Values one part of an address can hold: 256. RFC 791, the Internet Protocol: an address is four octets, and an octet is 8 bits, so each part runs from 0 to 255.
- Bits in an IPv4 address: 32. RFC 791: addresses are a fixed length of four octets, 32 bits. The prefix length counts how many of them name the network (RFC 4632, section 3.1).
- Addresses a block keeps back from devices: 2. RFC 1122, section 3.2.1.3: a host number of all zeros or all ones may not be given to a device, so the first address of a block names the network and the last is its broadcast. RFC 3021 lifts this for a /31 on a point-to-point link, and a /32 is one address on its own.
Why each step is the step it is
- The address you typed:
Math.floor(o1) * octet * octet * octet + Math.floor(o2) * octet * octet + Math.floor(o3) * octet + Math.floor(o4). The four parts put together as the one 32-bit number the address really is: each part is worth 256 times the part after it. - Addresses in the block:
Math.pow(2, addr_bits - p). The bits the prefix does not use are free to number devices, and each of them doubles the count: 2 to the power of (32 minus the prefix). - Usable host addresses:
size - reserved + reserved * tiny. Every address in the block less the network and broadcast addresses. A /31 keeps both of its two (RFC 3021) and a /32 is its one address. - Network address:
Math.floor(addr / size) * size. The address rounded down to a whole number of blocks, which is the same as setting every bit past the prefix to 0. - Broadcast address:
net + size - 1. The last address of the block, every bit past the prefix set to 1. A /31 and a /32 have no broadcast of their own, and this line then shows the block's last address. - Subnet mask:
octet * octet * octet * octet - size. The prefix's bits set to 1 and the rest to 0, which is 2^32 less the size of the block. - First usable address:
net + 1 - tiny. The address after the network address, or the network address itself on a /31 or a /32. - Last usable address:
net + size - reserved + tiny. The address before the broadcast, or the block's last address on a /31 or a /32. - Wildcard mask:
size - 1. The subnet mask with every bit flipped: 0 for the network's bits and 1 for the device bits.
What this cannot tell you
Every calculator has an edge, and a page that hides its edge is worse than no page at all. Here is this one's.
- It is arithmetic on the address and prefix you give it. It does not look at any network, and this site has not set up these networks itself.
- It works on IPv4 only. IPv6 addresses are 128 bits long and are not written this way.
- It cannot tell you whether an address is free, which device has it, or what your router hands out: a router's own settings page shows the range it gives out, which is often smaller than the block.
- On a /31 the two addresses are usable only on a point-to-point link between two devices whose software supports RFC 3021. On any other link, a /30 is the smallest block with two usable hosts.
- It does not say whether a range is private or public, or who it belongs to. Some ranges are set aside for special uses, and the registry that hands out a block is where to check one.
What a block that size is for
These follow from the answer above and nothing else. Each heading below covers one span of results, so read the one the calculator pointed you at and ignore the rest.
A link or a single address
A /30, a /31 or a /32: two usable addresses or one. Blocks this small connect two routers to each other or name one device, and they are not a network you would put a household on.
Room for one home network
From a /29 up to a /24, the 254 usable addresses that a 192.168.1.0/24 network holds. That is room for every phone, laptop, console and smart plug in an ordinary home, so when something will not connect, the address range is rarely the cause; the router and the Wi-Fi are the places to look. A single router covers a smaller home and a mesh system a larger one, and the two below are a router and a mesh system this site has written about.
Best WiFi Router for Home in 2026: 5 Ranked Picks covers this range in full.
TP-Link Archer BE230 Review 2026: Worth It for Wi-Fi 7? covers this range in full.
TP-Link Deco BE25 Review 2026: Cheap Wi-Fi 7 Mesh Worth It? covers this range in full.
Mesh Wi-Fi Setup 2026: 4 Best Kits and Node Placement Tips covers this range in full.
Bigger than one home network
From a /23 up to a /16. Networks this size belong to offices, schools and campuses, and they are usually split into smaller subnets by a managed router or switch. This site has not covered business networking gear, so it is not going to name any; a wired link is still the steadiest connection at any size.
Wireless vs Wired Network 2026: The Best Setup Is Both covers this range in full.
A block for a whole organisation
A /15 or larger, more than 65,534 usable addresses. Blocks this size are handed out by the regional internet registries to providers and large organisations, and are split into many smaller subnets before any device uses them.
If you have a mask rather than a prefix, the dropdown shows each prefix beside its mask, so pick the line that matches. A subnet calculator can only answer for the address and prefix you give it; to see what a PC is actually using, the ipconfig command on Windows lists both.
Questions about the subnet calculator
How does a subnet calculator find the network address?
It writes the address as one 32-bit number and keeps only the bits the prefix covers. 192.168.1.10 with a /24 prefix sits in a block of 256 addresses. The block starts at 192.168.1.0, the network address, and ends at 192.168.1.255, the broadcast address, and the 254 addresses between them, 192.168.1.1 to 192.168.1.254, are the ones devices can use. The same address with a /16 prefix is in 192.168.0.0, a block of 65,536.
How many usable hosts are in a /24 subnet?
254. A /24 leaves 8 bits for devices, which is 256 addresses, and the first and last are kept back as the network and broadcast addresses. The same rule gives a /26 64 addresses and 62 usable ones, and a /30 4 and 2, the smallest block that leaves two hosts on an ordinary link. Each extra bit in the prefix halves the block.
What subnet mask goes with a /24?
255.255.255.0. A mask is the prefix's bits set to 1 and the rest set to 0, so a /24 is 24 ones followed by 8 zeros. A /27 is 255.255.255.224, and a /16 is 255.255.0.0. The wildcard mask is the same bits flipped, which for a /24 is 0.0.0.255. The prefix box above lists every mask beside its prefix, so either one can be read from the other.
Why does a /31 have two usable addresses?
Because RFC 3021 lets a link between exactly two devices use both of them. A /31 holds 2 addresses, and the ordinary rule would keep both back as network and broadcast, leaving none. On a point-to-point link, where nothing needs a broadcast, the calculator counts 2: 192.168.1.0 and 192.168.1.1. A /32 is a single address, 192.168.1.10, used for one device or one route.
What does an IP subnet calculator need from me?
Any one address in the network and its prefix length, or its mask instead. On Windows, the ipconfig command lists a PC's IPv4 Address and Subnet Mask. An address of 192.168.1.10 with a mask of 255.255.255.0 is /24, which puts it in 192.168.1.0 with 254 usable hosts. Every address in the same block gives the same answer: 192.168.1.200 does too.
Sources
- RFC 791: Internet Protocol: Defines an IPv4 address as a fixed four octets, 32 bits, which is the one number every answer here is worked on.
- RFC 4632: Classless Inter-domain Routing (CIDR): Section 3.1 sets the prefix notation, an address followed by a slash and the number of significant bits from 0 to 32, and how it maps to a mask of ones followed by zeros.
- RFC 1122: Requirements for Internet Hosts: Section 3.2.1.3 rules out a host number of all zeros or all ones, which is why a block's first and last addresses are not given to devices, and names the all-ones address the directed broadcast.
- RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links: Lets both addresses of a /31 be used as host addresses on a link between two devices, which is the one exception to the two reserved addresses.
- Microsoft: ipconfig: The Windows command that shows a PC's own IPv4 address and subnet mask, the two figures this page takes.
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