Anybody who has manually configured their network settings has had to enter a subnet mask, usually in the form of “#.#.#.#”. The most familiar subnet value for many is 255.255.255.0, representing that the first three segments of the associated IP address must be the same as another IP address for the other address to match the subnet. The behaviour of this subnet is almost self-evident due to the syntax. However, this syntax can be misleading. What does a subnet of 255.240.0.0 mean? Or the subnet 255.192.0.0? And why is the subnet 255.127.0.0 invalid?

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