Subnetting in your head - What do we know?

What do we know about a /25?
128 mask
1 bits on and 7 bits off (10000000)
Block size of 128
2 subnets, each with 126 hosts

What do we know about a /26?
192 mask
 2 bits on and 6 bits off (11000000)
 Block size of 64
 4 subnets, each with 62 hosts

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

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

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

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 of whether you have a Class A, Class B, or Class C address, the /30 mask will provide you with only 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 “What Do We Know?” 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 networking 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 eventually anyway.