CIDR (Classless Inter-Domain Routing) is the modern notation for describing IP networks. It pairs an address with a prefix length (e.g. /24) indicating how many leading bits identify the network. It replaced the A/B/C class system.

How do I read 192.168.1.0/24?

The first 24 bits identify the network (192.168.1) and the last 8 are the host. The network has 256 addresses, of which 254 are usable (192.168.1.1–192.168.1.254). 192.168.1.0 is the network address and 192.168.1.255 is the broadcast.

Why does /31 give 2 usable hosts instead of 0?

In a regular /31 you'd only have 2 addresses and they'd be reserved as network and broadcast, leaving 0 hosts. RFC 3021 allows both addresses to be used as hosts on point-to-point links, so some implementations give 2 usable hosts on /31.

When would I use /32?

A /32 represents a single IP address — no network, no broadcast. It's useful for firewall/ACL rules pointing at one host, BGP advertisements of a single prefix, or application loopbacks in cloud.

Does this guide cover IPv6?

No. The guide and the associated calculator only cover IPv4. IPv6 uses a similar CIDR notation but with prefixes up to 128 bits and other particularities; it is out of scope for now.

CIDR subnetting guide for IPv4

Learn IPv4 CIDR and subnetting with /24, /30 and /32 examples, IP ranges, subnet masks and practical networking use cases.

What CIDR is

CIDR (Classless Inter-Domain Routing) is the modern notation for describing IP networks. A CIDR expression pairs an address with a prefix length (e.g. /24), which tells you how many leading bits identify the network. It replaced the rigid A/B/C class system, which wasted huge address blocks.

How to read a CIDR expression

Take 192.168.1.0/24. The number after the slash is the prefix length: how many of the IPv4 address bits (32 in total) belong to the network. With /24, the first 24 bits (192.168.1) identify the network and the remaining 8 bits are hosts inside it.

Network address: host bits all 0 (192.168.1.0).
Broadcast: host bits all 1 (192.168.1.255).
Usable hosts: 192.168.1.1 to 192.168.1.254 (254 addresses).
Subnet mask: 1s in network bits, 0s in host bits. For /24 it's 255.255.255.0.
Wildcard mask: bitwise complement of the mask. For /24 it's 0.0.0.255.

Special cases: /31 and /32

Two prefixes deserve special mention:

/31: traditionally only 2 addresses, both reserved as network and broadcast, leaving 0 usable hosts. RFC 3021 allows both addresses to be used as hosts on point-to-point links, which saves addresses on carrier networks.
/32: represents a single host. No network or broadcast. Useful for firewall/ACL rules targeting one IP, BGP advertisements, application loopbacks or per-host DNS entries.

Common subnet sizes

Usable hosts = 2^(32-prefix) − 2, except /31 (RFC 3021, 2 usable) and /32 (single host).

Prefix	Mask	Usable hosts	Typical use
/8	255.0.0.0	16 777 214	Very large network (classic class A).
/16	255.255.0.0	65 534	Wide space for campus/enterprise.
/24	255.255.255.0	254	Typical office/LAN subnet.
/25	255.255.255.128	126	Half of a /24.
/26	255.255.255.192	62	Quarter of a /24.
/27	255.255.255.224	30	Small department block.
/28	255.255.255.240	14	Smallest useful subnet for several hosts.
/29	255.255.255.248	6	Very small groups or DMZ.
/30	255.255.255.252	2	Classic point-to-point link.
/31	255.255.255.254	2 (RFC 3021)	Point-to-point with no reserved network/broadcast.
/32	255.255.255.255	1	Single host (application loopback, ACLs).

Compute your subnet with the CIDR calculator

Common mistakes

Assuming /24 has 256 usable hosts: it's 254 (network and broadcast aren't usable host addresses).
Confusing /25 (126 hosts) with two /25s combined (252 hosts).
Using 0.0.0.0/0 in a firewall rule without realising it matches ALL of the Internet.
Forgetting that /31 needs RFC 3021 support in old routers/OS stacks.
Mixing IPv4 and IPv6 masks: similar CIDR notation but different and incompatible systems.

Frequently asked questions

What is CIDR?: CIDR (Classless Inter-Domain Routing) is the modern notation for describing IP networks. It pairs an address with a prefix length (e.g. /24) indicating how many leading bits identify the network. It replaced the A/B/C class system.
How do I read 192.168.1.0/24?: The first 24 bits identify the network (192.168.1) and the last 8 are the host. The network has 256 addresses, of which 254 are usable (192.168.1.1–192.168.1.254). 192.168.1.0 is the network address and 192.168.1.255 is the broadcast.
Why does /31 give 2 usable hosts instead of 0?: In a regular /31 you'd only have 2 addresses and they'd be reserved as network and broadcast, leaving 0 hosts. RFC 3021 allows both addresses to be used as hosts on point-to-point links, so some implementations give 2 usable hosts on /31.
When would I use /32?: A /32 represents a single IP address — no network, no broadcast. It's useful for firewall/ACL rules pointing at one host, BGP advertisements of a single prefix, or application loopbacks in cloud.
Does this guide cover IPv6?: No. The guide and the associated calculator only cover IPv4. IPv6 uses a similar CIDR notation but with prefixes up to 128 bits and other particularities; it is out of scope for now.