What does CIDR notation (e.g. /24) mean in IP networking?

Classless Inter-Domain Routing (CIDR) notation specifies the number of leading 1-bits in the subnet mask. A /24 mask has 24 network bits (255.255.255.0) and 8 host bits, accommodating 256 total IP addresses (254 usable hosts).

Why are the first and last IP addresses in a subnet not usable for hosts?

In standard IPv4 subnets, the first address (all host bits 0) is reserved as the Network Identifier, and the last address (all host bits 1) is reserved as the Subnet Broadcast Address for broadcasting to all hosts in the segment.

What are the RFC 1918 Private IPv4 address ranges?

RFC 1918 reserves three address blocks for private internal networks: 10.0.0.0/8 (10.0.0.0 - 10.255.255.255), 172.16.0.0/12 (172.16.0.0 - 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 - 192.168.255.255). These are not routable on the public internet.

RFC 4632 Classless Inter-Domain Routing Engine

IPv4 Subnet & CIDR Calculator

Calculate network address, broadcast address, dotted-decimal subnet mask, wildcard mask, usable host ranges, and total address capacity for cloud VPCs and router subnets.

Network Engineering & CIDR Subnetting Guide

Classless Inter-Domain Routing (CIDR) & Subnet Arithmetic

In 1993, the IETF introduced Classless Inter-Domain Routing (CIDR / RFC 4632) to replace rigid legacy Class A, B, and C address allocations. Learn how subnet masks, wildcard masks, and variable length subnet masking (VLSM) divide IPv4 address spaces.

Subnetting Reference Table

IPv4 CIDR Prefix, Netmask & Host Capacity Reference

CIDR Prefix	Subnet Mask	Wildcard Mask	Total Addresses	Usable Hosts	Common Usage
/24	255.255.255.0	0.0.0.255	256	254	Standard home LAN, small office network
/25	255.255.255.128	0.0.0.127	128	126	Departmental corporate subnets
/26	255.255.255.192	0.0.0.63	64	62	Cloud VPC micro-tiers, Kubernetes nodes
/27	255.255.255.224	0.0.0.31	32	30	Cloud database cluster segments
/28	255.255.255.240	0.0.0.15	16	14	Datacenter server rack segments
/29	255.255.255.248	0.0.0.7	8	6	Public static IP block leased by ISP
/30	255.255.255.252	0.0.0.3	4	2	Point-to-point router interlinks
/32	255.255.255.255	0.0.0.0	1	1	Single host loopback / route rule

Class A Private

10.0.0.0/8

Provides 16,777,216 addresses (10.0.0.0 - 10.255.255.255). Standard for enterprise cloud VPC topologies (AWS VPC, GCP VPC, Azure VNet) and large corporate VPNs.

Enterprise VPC Architecture

Class B Private

172.16.0.0/12

Provides 1,048,576 addresses (172.16.0.0 - 172.31.255.255). Extensively used by Docker container bridge networking (docker0) and Kubernetes pod CIDRs.

Containers & Kubernetes

Class C Private

192.168.0.0/16

Provides 65,536 addresses (192.168.0.0 - 192.168.255.255). Divided into 256 /24 subnets. Standard for consumer Wi-Fi routers and local branch offices.

Home & Office LANs

Subnetting Knowledge Base

Frequently Asked Questions: Subnetting & CIDR Math

In standard IPv4 subnets, the formula for usable hosts is 2^(32 - prefix) - 2. The first IP address (where all host bits are binary 0) represents the Network Address identifying the segment. The final IP address (where all host bits are binary 1) is the Directed Broadcast Address used to transmit packets simultaneously to every host on that subnet. Neither can be assigned to a specific network interface.

A wildcard mask is the bitwise inverse of the subnet mask, calculated by subtracting each octet from 255. For example, a /24 subnet mask (255.255.255.0) has a wildcard mask of 0.0.0.255. In binary, a 0 indicates that the corresponding bit must match, while a 1 indicates "don't care." Wildcard masks are used in Cisco IOS Access Control Lists (ACLs) and OSPF routing configurations.

While standard RFC subnetting reserves 2 IP addresses, Amazon Web Services (AWS) reserves 5 IP addresses in every VPC subnet: 1) Network address (e.g. .0); 2) VPC router default gateway (.1); 3) Amazon-provided DNS resolver (.2); 4) Future AWS reserved use (.3); and 5) Broadcast address (.255). A /24 subnet in AWS provides 251 usable IP addresses instead of 254.