Subnets and CIDR
A /24 at the end of an address says how big the network is. This explains subnets, CIDR prefixes and netmasks, with the Subnet Calculator in Netutilus for Mac for the arithmetic.
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A subnet is a block of addresses that belong together: your home network, a company’s office, a provider’s range. Devices in the same subnet talk to each other directly. Anything outside it goes through the router.
Reading a prefix
An IPv4 address is 32 bits. A subnet is written as an address and a prefix, such as 192.168.1.0/24: the first 24 bits are the network, the same for every address in it, and the rest tell the devices apart.
The same split written as a mask is the netmask: 255.255.255.0 is /24, ones for the network bits and zeros for the rest.
Sizes worth knowing
- /24: 256 addresses, 254 usable. The usual home network.
- /16: 65,536 addresses.
- /30: 4 addresses, 2 usable, for a link between two routers.
- /32: one single address.
In IPv4 the first address names the network itself and the last is the broadcast address, so neither goes to a device. In IPv6 almost every network is a /64, more addresses than anyone will use.
Why it matters
- Two devices with addresses in different subnets can’t talk directly, even on the same Wi-Fi: a common reason a printer or NAS “disappears”.
- A VPN or a second router using the same range as your home network causes clashes.
Related
- Subnet Calculator, to work out any block.
- IP addresses.
Run it on your own network
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