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In binary notation, a subnet mask is represented by four octets, just like an IP address.

The following table shows the binary notation of the default subnet masks used in the class-based method.


When you use the binary notation of a subnet mask, you can change 32 numbers instead of the four numbers in the decimal notation. This improved capability provides a wider range of network sizes compared to the three fixed sizes in the class-based method.


Decimal notation Binary notation

  • 255.0.0.0 →  11111111 00000000 00000000 00000000
  • 255.255.0.0 → 11111111 11111111 00000000 00000000
  • 255.255.255.0 → 11111111 11111111 11111111 00000000