An IPv4 address has a network portion and a host portion; the subnet mask marks which bits are which — 1s mark network bits, 0s mark host bits.

* The mask's 1-bits mark the network portion, its 0-bits the host portion. *

IPv4 Address Structure:

The subnet mask determines which bits belong to which portion:

• Subnet mask bits set to 1 = Network portion
• Subnet mask bits set to 0 = Host portion

Example:

• IP: 192.168.10.10
• Subnet mask: 255.255.255.0
• Network portion: 192.168.10 (first 24 bits)
• Host portion: .10 (last 8 bits)

Key insight: The subnet mask is essential - without it, you cannot determine which portion is network vs host.

Go deeper:

• Subnet (Wikipedia) — the network/host boundary and subnet masks with binary examples.
• Subnetting Mastery (PracticalNetworking) — a 7-part series that builds the network/host split from first principles.

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.