|IP Addresses||Network Mask|
You can take those numbers and convert them into bits and count the bits. The number 255 converted to binary is 11111111. That is 8 bits. The whole IP address is 32 bits broken into groups of 8 bits. 255.255.255.0 would be converted to 11111111.11111111.11111111.00000000. If you look at the number you will notice that the number is a group of 1s followed by a group of 0s. We would call this a /24 network because there are 24 1s. All of the network masks can be converted into a group of 1s followed by a group of 0s. 255.255.255.128 would be converted into 11111111.11111111.11111111.10000000. This would be a /25 network because there are 25 1s in a row before the 0s.