I am taking my CCNA exam tomorrow for the second time, after failing it with the 825 last month. One of the questions was to do with how many subnets can you get out of using a 255.255.255.224 and the answers included both:
6
8
They did not specify whether the no ip subnet-zero was enabled or not - I went ahead and selected and answer that allowed the ip subnet-zero - what do you think is the correct answer?
When choosing a subnet mask, the mask value implies the number of host bits and the number of subnet bits. For host bits, the number of valid hosts per subnet is found using the formula 2n - 2, where n is the number of host bits. That formula is always true. To find the number of possible subnets, however, one of two formulas can be used, depending on a couple of factors. Choosing which formula to use can be tricky, and in part depends on the context and wording of the exam question. In short, if the question states or implies that classfull logic is needed, the formula should be 2n - 2, where n is the number of subnet bits. If the question implies the classless logic is needed, the formula is 2n.
Cisco makes no public statement about which to use if there are no hints in the question. In my humble opinion, if the question implies nothing at all about which formula to use, use 2n - 2. (Table 14-2 in Chapter 14, "Introduction to Dynamic Routing Protocols," introduces the concept of classless and classfull routing protocols. The ICND Exam Certification Guide covers more details about the differences.)
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