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Subnetting Class C Addresses


/27 What do we know about a /27? 224 mask 3 bits on and 5 bits off (11100000) Block size of 32 8 subnets, each with 30 hosts

/28 What do we know about a /28? 240 mask 4 bits on and 4 bits off Block size of 16 16 subnets, each with 14 hosts

/29 What do we know about a /29? 248 mask 5 bits on and 3 bits off Block size of 8 32 subnets, each with 6 hosts

/30 What do we know about a /30?

  • 252 mask

  • 6 bits on and 2 bits off

  • Block size of 4

  • 64 subnets, each with 2 hosts

Regardless whether you have a Class A, Class B, or Class C address, the /30 mask will only provide you with two hosts, ever. This mask is suited almost exclusively—as well as suggested by Cisco—for use on point-to-point links.

If you can memorize this section, you’ll be much better off in your day-to-day job and in your studies. Try saying it out loud, which helps you memorize things—yes, your significant other and/or coworkers will think you’ve lost it, but they probably already do if you are in the net- working field. And if you’re not yet in the networking field but are studying all this to break into it, you might as well have people start thinking you’re an odd bird now, since they will eventu- ally anyway.

It’s also helpful to write these on some type of flash card and have people test your skill. You’d be amazed at how fast you can get subnetting down if you memorize block sizes, as well as this “What Do We Know?” section.

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