The command ip subnet-zero allows the zero subnet to be used on a Cisco router. Does it also allow the all 1s subnet to be used ? Normally the number of available subnets is (2^n) - 2, where n is number of subnet bits. The two which are normally excluded are the all 0s and all 1s subnets. Does the above command include both of these ?
Pavlo got it exactly right (and I rated his post). The all ones (broadcast) subnet has always been available for use regardless of whether ip subnet-zero was enabled or not.
The all ones subnet and the all zero subnet are frequently thought of together (suggesting that they are related) but they are in fact quite separate in their functionality and in any restriction.
The point is also valid that ip subnet-zero has been enabled by default for a long time. I am sure that some study materials (and perhaps some certification tests) still put some emphasis on this. But in terms of practical networking the reality is that all subnets are available without making any effort.
The formula which subtracts 2 is the accurate formula for determining the number of hosts in a subnet. It also is sometimes found in some training materials (and who knows about some certification tests?). It represents an old (conservative) approach to network design. My perspective is that you would never go wrong by following it, but that little negative outcome will result if you do not follow it. I would hope that if this kind of question appeared on a test that both alternatives (subtract 2 or not subtract 2) would not show up as answer choices.