Today was an interesting lesson.  Today I learned how to do binary math and subnetting.  Chris made it quite easy to do the decimal to binary conversion and binary to decimal conversion.  No tricks, or anything difficult to remember.  One of the questions that was posed at the end of the lesson was:

“You have a IP of 165.10.0.0 and you need at least 150 subnets that have no more than 200 hosts apiece.”

So to start off with you know that the IP address given is a class B address, and the default subnet for class B is /16 (or 255.255.0.0).  So we setup our subnet by borrowing from the host bits.  Now if we were to only borrow 8 bits which would give us a subnet of /24 that would give us 256 valid subnets, however it would also give us 254 valid host addresses, which is more than the 200 maximum we were given.  So we just take one  more host bit and make our subnet mask /25 (or 255.255.255.128).  This gives us 512 valid subnets (2 to the power of 9).  Then that leaves us with 7 host bits which gives us 126 valid host addresses.  (2 to the power of 7 – 2).

I understand how it all works, it’s not all that difficult, the main thing to do is to practice a lot so that it becomes second nature.  Tomorrow I begin working on static routing.

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