What does /24 mean in CIDR?

Question

In CIDR notation, what does a /24 signify and roughly how many usable IPv4 addresses are in such a subnet?

Accepted Answer

In CIDR notation, the '/24' is a fundamental concept for defining subnet sizes and is crucial for efficient network design. It specifies the length of the network portion of an IP address, which in turn determines the number of available host addresses within that subnet. A /24 in CIDR notation signifies that the first 24 bits of the IPv4 address are allocated to the network portion, and the remaining 8 bits are reserved for host addresses. This corresponds to a subnet mask of 255.255.255.0, where the '255' represents 8 bits all set to '1'. To determine the total number of addresses, we use the formula 2 raised to the power of the number of host bits. With 8 host bits (32 total bits - 24 network bits), this means 2^8, which equals 256 total IPv4 addresses in the subnet. From these 256 total addresses, two are reserved and cannot be used for hosts: the network address (all host bits are zero) and the broadcast address (all host bits are one). Therefore, a /24 subnet provides 256 - 2 = 254 usable IPv4 addresses for devices on the network. Understanding CIDR notation like /24 is vital for network professionals to properly segment networks, manage IP address allocation, and ensure optimal routing efficiency. It's a foundational element in both network architecture and troubleshooting, ensuring robust and scalable network infrastructure.

What does /24 mean in CIDR?

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