If you practice this a few times you'll find it very easy to do in your head. To find the host range, simply add 1 to the final octet of the subnet number and subtract 1 from the final octet of the broadcast address.Īnd now you have your subnet number, broadcast address, and host range without having to use any binary. Lastly, anything to the right of your interesting octet will now change from 0 to 255. This gives you your interesting octet for the broadcast address. Simply take the interesting octet (80) and add the magic number (16) then subtract 1. You again deal with the interesting octet first, but you're now using the subnet number that we just found out. Simply place 80 in the interesting octet.įinding the broadcast address is very easy.
Guess what? We now have our subnet number.
Since our interesting octet in the IP address is 92 you'll find the multiples as follows.Īs you can see, 80 is the closest number to 92 without actually going over 92. This can be done easy by doubling the magic number until you have a number that is closest to/equal to but not more than the interesting octet in the IP address. Next you need to take the magic number and find the multiples of this number that is closest to or less than the interesting octet in the IP address. To find the magic number, simply subtract the number 256 from the interesting octet in the mask. There's nothing magic about it, but it has many uses. The first step is to calculate the magic number. This part is a little confusing at first, but will get easier with practice. (X means it's currently unknown)Īnything on the right of the interesting octet, signals that you should copy a 0 down into the subnet number.Īll that's left to find is the value of the interesting octet for our subnet number. Anything on the left of the interesting octet, signals that you should copy the IP address octet to your subnet number. Now that we have the interesting octet, compare the mask to the IP address. In our case, in red, shows the interesting octet. If the octet is neither a 255 or 0, that's your interesting octet. Take a look at each octet in the mask and see if it's a 255, 0, or some other number. It may be easier for you to stack the IP and mask as I've done above. First you need to find the interesting octet of the mask. Question: What's the subnet number, broadcast address, and host range of the below address? Practicing this method will allow you to find everything about a subnet in mere seconds. Since this is to help you answer questions fast, you should already have a basic understanding in subnetting. It makes subnetting quick and easy to figure out in seconds.
Lightbulb Quick Subnetting - All in your head! This probably isn't new for most of you guys, but I thought I'd share what my book taught me.
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