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?

This guide focuses on IPv4. The associated calculator also has an IPv6 mode for prefixes up to 128 bits, but the explanations about masks, broadcast, /31 and usable hosts on this page belong to IPv4.

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.

Next steps

CIDR answers the question “what IP ranges and subnets exist on my network”. The sibling question - “what devices are on the network and which vendor are they from” - is answered at the MAC layer. To understand how a MAC address and its OUI are read, and why a MAC does not geolocate a device, read the MAC address and OUI guide or jump straight to the MAC vendor and OUI lookup to identify a specific device on the local network.

To keep both calculators within reach, the networking tools hub groups CIDR and the MAC lookup with cheat sheets and examples.

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?: This guide focuses on IPv4. The associated calculator also has an IPv6 mode for prefixes up to 128 bits, but the explanations about masks, broadcast, /31 and usable hosts on this page belong to IPv4.