From the course: Cisco Certified Support Technician (CCST) Networking Cert Prep

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Calculating available subnets

Calculating available subnets

From the course: Cisco Certified Support Technician (CCST) Networking Cert Prep

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Calculating available subnets

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(gentle music) - [Instructor] If we decide that we want to do subnetting, we want to add bits to a network's default subnet mask so that we can take one network address space and chop it up into different subnets. Let's do the math to determine how many subnets do we create if I add a certain number of bits to a default subnet mask. And here's the magic formula. The number of subnets we create can be determined by the formula, two raised to the power of s, where s is the number of borrowed bits. And that is a big term to write down in your notes. A borrowed bit is a bit that is added on to a default subnet mask. Now, to know what the default subnet mask is, you need to be able to distinguish between a Class A and a Class B and a Class C address. Do you remember how you do that? You look at the first octet. And if that first octet has a number in the range of one through 126, that's Class A, and it has a /8 subnet mask by default. A Class B network is going to have 128 through 191 in…

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