What is the difference between CIDR and subnet mask?

CIDR notation (/24) and subnet mask (255.255.255.0) represent the same information in different formats. A /24 prefix means 24 bits are 1 in the subnet mask, which gives 255.255.255.0. CIDR is more compact and universally preferred in modern networking.

What replaced classful IP addressing?

CIDR (Classless Inter-Domain Routing), introduced in 1993 via RFC 1519, replaced classful IP addressing. Classful addressing divided IPv4 into fixed Class A (/8), Class B (/16), and Class C (/24) networks, which was wasteful. CIDR allows any prefix length from /0 to /32.

Fundamentals

What Is CIDR Notation?

Q: What does the slash mean in an IP address?

The slash (/) in CIDR notation separates the network address from the prefix length. For example, in 192.168.1.0/24, the /24 means the first 24 bits of the address identify the network, leaving 8 bits for host addresses within that network.

A plain-English explanation of CIDR notation — what the slash number means, how to read it, and why it replaced classful addressing.

The anatomy of a CIDR block

CIDR notation combines an IP address with a prefix length, separated by a slash:

192.168.1.0 / 24

■ Network portion (24 bits) ■ Host portion (8 bits)

→ The IP address (192.168.1.0) is the starting point of the range — the network address.
→ The prefix length (/24) tells you how many of the 32 address bits are fixed — identifying the network.
→ The remaining bits (32 − 24 = 8) are the host bits — they can vary to identify individual hosts.
→ 8 host bits = 2⁸ = 256 total addresses, with 254 usable (the first is the network address, the last is the broadcast).

CIDR vs. classful addressing

Before CIDR, IPv4 addresses were divided into fixed "classes":

Class	Range	Prefix	Hosts	Problem
A	1.0.0.0 – 126.255.255.255	/8	16 million	Massive — wasteful for all but huge orgs
B	128.0.0.0 – 191.255.255.255	/16	65,534	Still large — most orgs didn't need 65K IPs
C	192.0.0.0 – 223.255.255.255	/24	254	Often too small — needed multiple /24s

CIDR (RFC 1519, 1993) replaced this rigid system with flexible prefix lengths. A network can now be exactly as large as needed: /22 for 1,022 hosts, /26 for 62 hosts, or any size in between. This dramatically reduced IPv4 address waste.

Reading a CIDR block

Given 10.0.0.0/8, you can calculate:

Prefix length	/8 (8 bits fixed)
Host bits	32 − 8 = 24 bits
Total IPs	2²⁴ = 16,777,216
Usable hosts	16,777,214
Network address	10.0.0.0
Broadcast address	10.255.255.255
Subnet mask	255.0.0.0

Calculate 10.0.0.0/8 →

Frequently Asked Questions

What does the slash mean in an IP address?

The slash separates the network address from the prefix length. In 192.168.1.0/24, the /24 means the first 24 bits identify the network, leaving 8 bits for host addresses.

What is the difference between CIDR and a subnet mask?

They represent the same information in different formats. /24 is equivalent to the subnet mask 255.255.255.0. CIDR notation is more compact and is used everywhere today; subnet masks are mostly encountered in older equipment and documentation.

Is /24 the same as 255.255.255.0?

Yes — they are two ways to express the same subnet. /24 means 24 bits are set to 1 in the subnet mask, which in dotted-decimal is 255.255.255.0.