However, classful subnet selection is not longer relevant today (A, B, C...) The Mac UI limited you to those three classful subnet masks.
Today, it's much simpler to type /26 after your IP address instead of entering 255.255.255.192, or playing around with a non-standard host/network slider.
> The Mac UI limited you to those three classful subnet masks.
And it assumed a specific network architecture -- e.g, if you have a class A network, the middle two octets of your address represent a "subnet" and the last octet represents a "node". This may have been appropriate in pre-CIDR networks where all local nets were /24, but it's no longer true today; it's not uncommon to have networks which split on non-octet boundaries.
Today, it's much simpler to type /26 after your IP address instead of entering 255.255.255.192, or playing around with a non-standard host/network slider.