Dictionary · Networking Fundamentals
Subnet Mask
Addressing · Beginner
A 32-bit value that marks which part of an address is the network and which part identifies the host. A mask of 255.255.255.0 leaves 8 bits for hosts.
The mask tells a host which destinations are local and which must be sent to a gateway. Two hosts can share the same numbers and still be on different networks if their masks differ.
Picture it
255.255.255.0 marks the same split as /24. It tells the host which destinations are local. It is not the host's own identity.
Example: With a mask of 255.255.255.0, 192.168.10.5 and 192.168.10.40 are on the same network. 192.168.11.5 is not.
Common misconception
- The mask is not an address you assign to a device as its identity. It describes how to read the address.