Notice how the Most Significant Bit (weight 128) is permanently locked to 0. Toggle the remaining 7 bits to see how the maximum possible decimal value is capped at 127.

Locking the Most Significant Bit (Class A) For a Class A network, the rule is that the very first bit—the Most Significant Bit on the far left—must always be 0. By forcing that 128-value bit to stay off, the highest binary number you can possibly create in that octet is 01111111. If you add up the remaining 7 bits (64 + 32 + 16 + 8 + 4 + 2 + 1), the maximum total is 127. Locking the MSB neatly splits the entire 256-number range perfectly in half, reserving the entire bottom half (0 through 127) exclusively for Class A networks. What if they locked the Least Significant Bit instead? If they had locked the LSB (the 1 value on the far right) to 0 instead, the results would be chaotic for routing. Changing the MSB changes the number in massive chunks, but changing the LSB just changes whether a number is odd or even. If the LSB was locked to 0, your available networks would be 0, 2, 4, 6, 8... all the way up to 254. Grouping networks by "only even numbers" would make routing tables a nightmare to organize efficiently.