What is CIDR Notation? Subnetting and Prefix Lengths Explained

When inspecting IP addresses and network allocations, you will frequently see an address followed by a forward slash and a number, such as 192.168.1.0/24 or 10.0.0.0/16.

This representation is called CIDR notation (Classless Inter-Domain Routing). Introduced in 1993 by the IETF (RFC 1519), CIDR replaced the rigid, wasteful “classful” networking system (Class A, B, and C) that threatened to deplete Internet addresses in the early 1990s.

The Anatomy of CIDR Slash Notation

In CIDR notation, the number after the forward slash represents the prefix length: the exact number of leading bits that represent the network portion of the address.

Because an IPv4 address has 32 bits: $$\text{Host Bits} = 32 - \text{Prefix Length}$$ $$\text{Total Addresses} = 2^{\text{Host Bits}}$$ $$\text{Usable Hosts} = 2^{\text{Host Bits}} - 2$$

(In standard IPv4 subnets, 2 addresses are reserved: the network address with all host bits set to 0, and the broadcast address with all host bits set to 1).

Common IPv4 CIDR Reference Table

CIDR Prefix	Subnet Mask	Total Addresses	Usable Host IPs	Common Application
/32	255.255.255.255	1	1	Single host / firewall rule
/30	255.255.255.252	4	2	Point-to-point router links
/29	255.255.255.248	8	6	Small business static IP blocks
/28	255.255.255.240	16	14	Small corporate subnet
/24	255.255.255.0	256	254	Standard home or office LAN; smallest BGP advertisement
/20	255.255.240.0	4,096	4,094	Mid-sized enterprise campus
/16	255.255.0.0	65,536	65,534	Cloud VPC network (e.g. AWS default)
/8	255.0.0.0	16,777,216	16,777,214	Massive tier-1 ISP or government block

Why the /24 Prefix Matters in BGP Routing

If you look up public IP addresses, you will notice that almost every public network route has a prefix length of /24 or shorter (e.g., /22, /20, /16).

This is because the global Internet routing table enforces a minimum prefix size rule: most tier-1 transit providers will ignore BGP route advertisements smaller than a /24 (such as /25 or /28). Without this restriction, hundreds of millions of micro-subnets would flood and crash core Internet router memory.

CIDR in IPv6

IPv6 also uses CIDR notation, but with 128 bits:

• /32: Standard allocation block granted to an Internet Service Provider by an RIR.
• /48: Standard enterprise or business site allocation.
• /64: Standard single subnet (e.g. one local home network). A single /64 contains $18,446,744,073,709,551,616$ individual addresses!
• /128: A single host interface.