Can someone please explain to me an inconsistency in the ICND1 Lab Guide concerning the calculation of usable subnets? I emphasize “usable” since Cisco does.
Lab 4-3, Task 4 states that we are assigned the address space 192.168.111.0 /28. Q2 asks how many subnets result using this mask/prefix. The answer given is 16, or 2 to the 4th power. Now look at Task 5: 172.25.0.0/23, number of subnets = 126, or 2 to the 7th power minus 2. So why “minus 2” here and not in Task 4? Same applies to Task 6: 172.20.0.0/25, number of subnets = 510, or 2 to the 9th power minus 2.
I learned that the zero subnet is not allowed in theory, unless explicitly configured to be allowed in praxis in a working network; that the ‘broadcast’ (last mathematically possible) subnet is never allowed; and therefore to assume “2 to the nth power minus 2” (or “minus one” if the zero-subnet is explicitly to be allowed).
This inconsistency in the documentation has serious consequences for those taking the CCNA exam. What am I to tell my students? Is the formula “2 to the nth power minus 2” correct, or is it “ 2 to the nth power”? TestKing is no help on this either, as their exam prep simulator also indiscriminately uses both formulas, sometimes allowing, sometimes disallowing, both the zero and ‘broadcast’ subnets.