What is CIDR and subnetting?
CIDR describes how much of an address represents the network and how much represents the host portion. It is the modern notation behind prefixes like /24 and /64.
- How prefixes work.
- Why a /24 differs from a /32 or /64.
- How subnetting helps routing and policy design.
CIDR basics
A prefix length says how many leading bits belong to the network.
In IPv4, /24 is common for LAN-style blocks. In IPv6, /64 is the standard subnet size for many designs.
Why it matters
Subnetting controls how many addresses are in a block and what is considered on-link.
It also affects ACLs, summarization, and route policy.
Practical mistakes
People often confuse a host IP with a CIDR network.
Another mistake is using an invalid prefix size for the address family.
How WhenIP helps
The subnet calculator gives you quick structure without leaving the page.
That is useful when checking LAN ranges, firewall rules, or documentation.
Is /32 always one device?
For IPv4 it usually means one host route. In IPv6, /128 is the single-address equivalent.
Why is /64 so common in IPv6?
Because many IPv6 mechanisms assume a /64 subnet size.
Last updated: March 29, 2026