Subnet Zero

For questions on subnetting please go to the Main Page

In the past, this website also provided questions relating to old routing kit that did not allow the use of the all "1" subnet or all the "0" subnet (known as 'subnet zero'). The majority of site users are revising for cisco exams so the questions have been removed to avoid confusion.

On this website, and on all new cisco routers, the number of subnets is calculated using the formula of 2^n by default. However, for legacy routers network engineers use 2^n - 2 to comply with the almost obselete RFC 1878 standard. Some text books and courses that relate to old routing equipment also refer to the old formula as do some Microoft Learning courses. If you're planning to take a non-Cisco exam that involves subnetting you should make sure you understand the 'subnet zero' concept' and check what is considered the default correct formula by your examining board. Cisco's position on subnet zero is stated here.

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