Networking
CIDR
Also known as: Classless Inter-Domain Routing, Slash Notation, Subnet Notation
The notation for IP address ranges — 192.168.1.0/24 means the first 24 bits are the network, leaving 256 host addresses in that block.
CIDR (Classless Inter-Domain Routing) is the notation used to describe IP address ranges and subnet sizes. A CIDR notation like 192.168.1.0/24 means the first 24 bits are the network address, leaving 8 bits for host addresses — 256 addresses in that subnet.
Before CIDR, IP networks were divided into fixed classes (A, B, C) with fixed sizes — wasteful and inflexible. CIDR replaced this with variable-length subnet masks, letting you carve up IP space precisely.
The slash number (prefix length) tells you how many bits are fixed for the network portion:
- /24 — 256 addresses (254 usable). The most common subnet for a single office VLAN.
- /16 — 65,536 addresses. Typical for larger enterprise networks using the 10.0.0.0/8 range.
- /30 — 4 addresses (2 usable). Often used for point-to-point links between routers.
- /32 — a single host address. Used in routing policy and firewall rules to reference a specific device.
You'll encounter CIDR notation constantly when configuring firewalls, defining DHCP scopes, writing ACLs, and reading routing tables. "Allow traffic from 10.10.0.0/16" means allow any source address in the 10.10.0.0 to 10.10.255.255 range.
One quick mental model: each bit you add to the prefix length halves the subnet. A /24 has 256 addresses. A /25 has 128. A /26 has 64.