2026-09-20 · IPV4 Scanner
Subnetting has a reputation for being hard. It isn't, really — it is one idea repeated, and the reason it feels difficult is that it is usually taught as binary arithmetic before anyone explains what it is for.
So: what it is for first.
An IP address has two parts: which network you are on, and which host you are within it. The split is not fixed. The notation /24 says the first 24 bits identify the network and the remaining 8 identify the host. That is all CIDR notation means — a count of how many leading bits are the network part.
So in 192.168.1.0/24, the network is 192.168.1 and the last number is yours to allocate. Change it to /16 and the network becomes 192.168, leaving two numbers to allocate — a much bigger network.
The number after the slash is network bits, out of 32. Whatever is left is host bits, and host bits determine size.
| CIDR | Subnet mask | Total addresses | Usable hosts |
|---|---|---|---|
| /30 | 255.255.255.252 | 4 | 2 |
| /29 | 255.255.255.248 | 8 | 6 |
| /28 | 255.255.255.240 | 16 | 14 |
| /27 | 255.255.255.224 | 32 | 30 |
| /26 | 255.255.255.192 | 64 | 62 |
| /25 | 255.255.255.128 | 128 | 126 |
| /24 | 255.255.255.0 | 256 | 254 |
| /23 | 255.255.254.0 | 512 | 510 |
| /22 | 255.255.252.0 | 1024 | 1022 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
The pattern is simple: each step down doubles the size. Memorise /24 = 256 addresses and you can work out the rest by halving or doubling.
Every subnet loses two. The first address is the network address, which names the subnet itself. The last is the broadcast address, which reaches every host on it. Neither can be assigned to a device.
So a /24 has 256 addresses and 254 usable ones. A /30 has four addresses and two usable, which is exactly why /30 is the traditional choice for a point-to-point link between two routers: two usable addresses, two ends, nothing wasted.
The question that actually comes up is: given an address and a prefix, where does the subnet start and end?
Take 192.168.1.130/26. A /26 has 64 addresses, so subnets begin at multiples of 64: .0, .64, .128, .192. The address 130 falls in the block starting at 128. So the network address is 192.168.1.128, the broadcast is 192.168.1.191 (one before the next block at 192), and the usable range is .129 to .190.
The general method: find the block size (256 minus the last octet of the mask), then find the largest multiple of the block size at or below your address. That is the network address. Add the block size, subtract one, and you have the broadcast.
The subnet calculator does all of this instantly, and there is no virtue in doing it by hand when you are configuring something real. Understanding the mechanism matters; performing it manually does not.
Broadcast containment. Broadcast traffic reaches every host in a subnet. Put two thousand devices on one flat network and every one of them processes every broadcast, which wastes capacity and slows everything down.
Security boundaries. Subnets are where you apply rules. Putting guest Wi-Fi, security cameras and staff laptops on separate subnets lets you stop the cameras talking to the laptops — which matters, because network cameras are among the least well-maintained devices anyone owns.
Structure. Reading a network diagram is far easier when the addressing means something.
Three blocks are reserved for internal use and are never routed on the internet:
10.0.0.0/8 — 16.7 million addresses172.16.0.0/12 — 1 million addresses, covering 172.16 to 172.31192.168.0.0/16 — 65,536 addressesHome routers almost always use a /24 out of 192.168. The 10 range is the usual choice for larger networks because it leaves room to structure things by site or department.
One practical warning: if you use 192.168.1.0/24 at home and connect to an office VPN that also uses 192.168.1.0/24, routing becomes ambiguous and things break in confusing ways. Choosing an unusual range for your own network avoids a genuinely annoying problem.
What does the /24 in an IP address mean?
That the first 24 bits identify the network and the remaining 8 identify hosts within it, giving 256 addresses of which 254 are usable.
Why can't I use the first and last address?
The first identifies the subnet itself and the last is the broadcast address for it. Both are reserved by the protocol.
What is the difference between a subnet mask and CIDR notation?
They express the same thing. 255.255.255.0 and /24 are identical; CIDR is just shorter.
How many hosts in a /26?
64 addresses, 62 usable. Each step down from /24 halves the size: /25 is 126 usable, /26 is 62, /27 is 30.
Can I use any private range I like?
Yes, within the three reserved blocks. Pick something unlikely — 192.168.73.0/24 rather than 192.168.1.0/24 — so it does not collide with a VPN or another network you connect to.