Read an address block: what /24 really means
Network settings are full of things like 192.168.1.0/24 and 255.255.255.0. They look like a secret code, but they answer one simple question: which addresses belong to the same network as you? In this lesson you’ll find your own network’s block, learn to read it, and work out how many devices it can hold, without any maths beyond counting.
In this lesson
- Find your own subnet mask and turn it into a /number
- Read a block like 192.168.1.0/24: where it starts, where it ends, how many devices fit
- Tell whether two addresses are on the same network
You’ll need: the address and subnet mask from Lesson 2, and our subnet calculator to check your answers. Works on a phone too.
Why networks come in blocks
Think of a street. The house numbers on one street are a block: 1 to 100, say. If you want to deliver to a house on the same street, you walk there yourself. If it’s on a different street, you hand the parcel to the post office.
Your devices do the same. Every time your laptop sends something, it first asks: is this address in my own block? If yes, it sends it directly across your home network, to the printer for example. If not, it hands it to the router, the “post office”, to send out to the internet. The subnet is that block, and the subnet mask or the /number is how a device knows where its block ends.
Try it yourself · part 1
Find your own block
- Find your address and subnet mask the way you did in Lesson 2. On Windows, run
ipconfigand look at the IPv4 Address and Subnet Mask lines. On a Mac it’s in Wi-Fi → Details… → TCP/IP; on iPhone, in the Wi-Fi network’s ⓘ page. - For most homes you’ll see a mask of
255.255.255.0. - Open the subnet calculator and enter your address with
/24after it, for example192.168.1.23/24. - Look at the network address, the first and last usable address, and the number of hosts. That’s your home network’s whole block.
Reading 192.168.1.0/24
An IPv4 address is four numbers from 0 to 255. Behind the scenes it’s 32 ones and zeros (bits), eight for each number. The /24 means: the first 24 bits, the first three numbers, are the network’s “street name”, shared by every device on it. Only the last number changes from device to device.
- Network:
192.168.1.0, the street name itself. It isn’t given to any device. - Devices:
192.168.1.1to192.168.1.254. That’s 254 usable addresses. The router usually takes one of them, often .1. - Broadcast:
192.168.1.255, a special address that means “everyone on this network”. Not given to any device either.
So a /24 has 256 addresses in total, of which 254 can be used by devices. That’s plenty for a home, which is why nearly every router uses one.
Mask and /number are the same thing
The subnet mask 255.255.255.0 is just another way of writing /24. Each 255 is eight bits “switched on” for the network part: three 255s are 24 bits. The table shows the blocks you’ll meet most often:
| /number | Subnet mask | Addresses | Usable by devices |
|---|---|---|---|
| /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 | 1,024 | 1,022 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
Notice the pattern: every step down in the /number doubles the size of the block. Every step up halves it. A bigger number after the slash means a smaller network.
Try it yourself · part 2
Same network or not?
Work these out in your head first, then check each one in the subnet calculator.
- 192.168.1.20 and 192.168.1.200, both /24. The first three numbers match and the block covers .1 to .254, so yes, same network.
- The same two addresses, but /25. A /25 is half a /24, so 192.168.1.0/25 covers only .0 to .127, and 192.168.1.128/25 covers .128 to .255. Now .20 and .200 are on different networks, and to reach each other they’d have to go through a router.
- 192.168.1.23 and 192.168.2.23, both /24. The third number differs, so they’re on different networks, even though the last numbers match.
To see the second one for yourself, enter 192.168.1.0/24 in the calculator and choose Split this network into 2 subnets: you get the two /25 halves, with the first and last host of each.
Splitting one block into smaller ones like this is what subnetting means. Offices do it to keep, say, staff, guests and security cameras on separate networks. At home, the guest Wi-Fi on many routers works the same way.
Blocks you already know
The private ranges from Lesson 2 are just three big blocks, written this way:
| Block | Covers | Where you meet it |
|---|---|---|
| 10.0.0.0/8 | 10.0.0.0 to 10.255.255.255 | Offices, some home routers, phone hotspots |
| 172.16.0.0/12 | 172.16.0.0 to 172.31.255.255 | Offices and some routers |
| 192.168.0.0/16 | 192.168.0.0 to 192.168.255.255 | Most home routers, which use one /24 slice of it |
| 100.64.0.0/10 | 100.64.0.0 to 100.127.255.255 | Shared addresses for CGNAT, from Lesson 3 |
Now the “172.16 to 172.31” rule from Lesson 2 makes sense: it’s simply where a /12 block starting at 172.16 ends.
Where you’ll see these in real life
- Your router’s settings (Lesson 3): the LAN or DHCP page shows the router’s address and mask, and the range it hands out has to fit inside that block.
- Who owns an address: internet providers and companies are given blocks too. Our ASN lookup shows the blocks a network announces, often hundreds of them.
- Firewall and VPN settings: “allow 192.168.1.0/24” means “allow every device on that home network”.
/64: so many addresses that nobody ever has to count them. The idea is identical: the number says how much of the address is the network part.Key takeaways
- A subnet is a block of addresses that can reach each other directly; anything outside it goes through the router.
- The /number says how many of the address’s 32 bits are the network part. /24 is the same as the mask 255.255.255.0.
- A /24 has 256 addresses: the network address, 254 for devices, and the broadcast address.
- Each step down in the /number doubles the block; each step up halves it.
- The private ranges are just blocks: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16.
Check yourself
Three questions. Pick one answer each and you’ll see why.
1. Your laptop shows the subnet mask 255.255.255.0. How is that written as a /number?
2. How many devices can have an address on a /24 network?
3. With a /25 mask, are 192.168.1.20 and 192.168.1.200 on the same network?
Want the background in more depth? Read What Is a Subnet?, the full explainer this lesson builds on.
This lesson is part of the free course How the Internet Works, licensed under CC BY-NC-SA 4.0: you may share and adapt it for non-commercial use, with credit to ExamineIP and a link to the original.