Subnet Calculator
Enter an address in CIDR notation to see the network, the broadcast address, the usable range and how many hosts fit. Everything is computed in your browser.
Binary view
Split this network
Dividing it into equal subnets:
| Prefix | Subnets | Hosts each | First subnet |
|---|
Common prefixes: /8 · /16 · /20 · /22 · /23 · /24 · /25 · /26 · /27 · /28 · /29 · /30 · /31 · /32 · all prefixes
What the prefix length means
An IPv4 address is 32 bits. The number after the slash says how many of those bits identify the network; the rest identify a host within it. In 192.168.1.0/24 the first 24 bits are the network, leaving 8 bits — 256 addresses — for hosts.
Each bit you take away from the prefix doubles the size of the network. A /23 is twice a /24; a /25 is half. That doubling is why subnet sizes are always powers of two and why you cannot have a network of exactly 100 addresses — you take a /25 and leave 28 addresses unused.
Why two addresses are unusable
In every subnet larger than a /31, the first and last addresses are reserved. The first, with all host bits zero, is the network — it names the subnet rather than a machine in it. The last, with all host bits one, is the broadcast address, which reaches every host at once.
So a /24 holds 256 addresses but only 254 usable ones. This is the arithmetic behind the constant off-by-two in capacity planning, and it gets painful at the small end: a /30 has four addresses and only two you can assign, which is why point-to-point links traditionally waste half their space.
Two prefixes are special. A /31 would have no usable addresses under that rule, so RFC 3021 redefines it for point-to-point links: both addresses are usable, and there is no broadcast. A /32 is a single address — a host route, a loopback, or a firewall rule matching exactly one machine.
The mask and the wildcard
The same information is written three ways depending on which tool you are talking to. A prefix length of 24 is the subnet mask 255.255.255.0, and its inverse — the wildcard mask 0.0.0.255 — is what Cisco access lists and OSPF statements expect. A wildcard is simply the mask with every bit flipped, so a bit that is 0 in the wildcard must match and a bit that is 1 is ignored.
Private ranges
Three blocks are reserved for private use and are never routed on the public internet:
| Range | CIDR | Addresses | Where you see it |
|---|---|---|---|
| 10.0.0.0 – 10.255.255.255 | 10.0.0.0/8 | 16,777,216 | Large corporate networks and cloud VPCs |
| 172.16.0.0 – 172.31.255.255 | 172.16.0.0/12 | 1,048,576 | Docker's default bridge, mid-size networks |
| 192.168.0.0 – 192.168.255.255 | 192.168.0.0/16 | 65,536 | Home routers, almost universally |
The 172 range trips people up because it is not the whole of 172.x: it stops at 172.31, so 172.32.0.0 is public address space belonging to someone else.
Ranges worth recognising
| Block | What it is |
|---|---|
127.0.0.0/8 | Loopback — the whole /8, not just 127.0.0.1 |
169.254.0.0/16 | Link-local. An address here usually means DHCP failed |
100.64.0.0/10 | Carrier-grade NAT. Appears in mobile networks and some ISPs |
224.0.0.0/4 | Multicast |
0.0.0.0/0 | The default route — every address, which is why it means "anywhere" in a firewall rule |
Frequently asked questions
How many hosts are in a /24?
256 addresses, of which 254 are usable. The first is the network address and the last is the broadcast address, and neither can be assigned to a machine.
What is the subnet mask for /24?
255.255.255.0. The prefix length counts the leading 1 bits in the mask, so 24 ones followed by 8 zeros gives three full octets of 255 and one of 0.
Is a smaller prefix number a bigger network?
Yes, and this is the usual source of confusion. The number counts network bits, so fewer bits leaves more for hosts. A /16 is 256 times larger than a /24.
Why can a /31 be used when it has no usable hosts?
Under the normal rule a /31 would be entirely network and broadcast. RFC 3021 makes an exception for point-to-point links, where there is no need for a broadcast address, so both addresses are assignable.
What is a wildcard mask?
The bitwise inverse of the subnet mask, used by Cisco access lists and OSPF. For a /24 the mask is 255.255.255.0 and the wildcard is 0.0.0.255. A zero bit must match; a one bit is ignored.
How do I split a /24 into smaller subnets?
Each extra prefix bit halves the network. A /24 splits into two /25s, four /26s, eight /27s and so on. The calculator above shows the first subnet at each size.