What is the prefix length notation and how does it relate to the subnet mask?

Prefix length is the count of 1-bits in the subnet mask, written as a slash (e.g. /24 = 255.255.255.0); it is just a compact way to write the mask.

* Each longer prefix adds one network bit, lengthening the mask. *

Prefix Length (Slash Notation): A shorthand way to express the subnet mask by indicating the number of bits set to 1.

Example:

• 192.168.10.10/24 means:
  • IP address: 192.168.10.10
  • Subnet mask: 255.255.255.0
  • First 24 bits are network, last 8 bits are host

Context: Prefix notation is more concise and commonly used in network documentation and router configurations.

Go deeper:

• Classless Inter-Domain Routing — CIDR notation (Wikipedia) — where the slash notation comes from and how the prefix counts leading 1-bits.
• ipcalc (jodies.de) — type a prefix and watch it expand to the binary mask, network, and host range.