GLOSSARY · CIDR NOTATION, NETWORK PREFIX
Subnet and CIDR
A subnet is a block of consecutive IP addresses treated as one network. CIDR notation describes it with a starting address and a slash number, such as 192.168.1.0/24, where the number says how many leading bits identify the network. The remaining bits identify individual devices.
Why it matters
Subnets decide which devices can talk to each other directly and how addresses are handed out and routed. The size follows from the prefix: a /24 has 256 addresses, a /16 has 65,536, and a smaller prefix means a bigger block. Internet providers announce their ranges in CIDR form, and firewalls, blocklists and allowlists use the same notation to cover many addresses in one line.
In practice
When a provider says a range such as 203.0.113.0/24 is blocked, every address in the 256 is affected. For IPv6, prefixes are longer: a /64 is the normal size of one local network. The subnet calculator converts a prefix into its first and last address, mask, count of usable hosts and a binary view, for both IPv4 and IPv6.
Related terms
- IPv4: IPv4 is the older and still most common version of the internet's addressing system.
- IPv6: IPv6 is the newer version of the addressing system, built to replace IPv4.
- ASN: An ASN is a unique number assigned to a network that manages its own routing on the internet, such as an internet provider, a cloud company, a university or a large business.
- Public vs. private IP address: A public IP address is reachable across the internet and is the one websites see.