Networking is exciting, and the more I learn about networking the more I realize it's a profession that requires a lot of training and studying to become really skilled in.

While I was studying for the CCNA I created this simple tool to help me drill binary conversions.


How it works

The prefix2netmask app is a certification preparation tool to help students get exposure and training in binary conversion from a CIDR prefix to a subnet mask, or from a subnet mask to a CIDR prefix.

To submit an answer the student can press the enter key, or press the "Check!" button.

If the answer is correct a new task will be presented and the input field will be cleared. If the answer was incorrect the input field will glow red, and the student can retry. A new task will only be presented if a correct answer is provided.

Under the input field is a scoreboard. This will show how many correct answers were given compared to how many answers have been given.


There are three difficulty levels for each conversion. One for each network class.

  • Level 1 will only provide Class C subnet masks
  • Level 2 will only provide Class B subnet masks
  • Level 3 will only provide Class A subnet masks
  • Random will provide subnet masks from all three levels

Changing difficulty will not reset the counters.


When "Prefix → network mask" is selected, the student will be presented with an IP address suffixed with CIDR prefix annotation. If this Class A IP address is presented as task:


The correct subnet mask and answer is:

When "Network mask → prefix" is selected, the student will be presented with an IP address and a subnet mask. If the following information is provided:

IP address:
Network mask:

The correct CIDR prefix and answer is: 28

Note: The forward-slash (/) should not be provided in the answers.