The 7 second subnetting method
I was today years old when I learned Professor Messer's "7 second subnetting method". It blew my mind, because instead of the manual "convert to binary, do lots of math" method I learned, you can rely on two simple charts that use nothing more than addition and subtraction. This is great for exam situations as well, as you can easily create the charts on scratch paper in under 2 minutes.
- Create chart one: CIDR/Mask to decimal, number of networks, and number of total addresses. This sounds difficult, but if you can remember where to put the 1(!), 2(!!), and 255(!!!) the hardest part is some addition to get the decimal conversion.
Chart is 7*8. Columns 1-4 are your CIDR or netmasks. Column 5 is your decimal conversion (Start from the bottom, and subtract the number of addresses in your next row from the prior rows decimal). Six is the number of networks, and 7 is the number of addresses.
- Create chart 2: Network address subnet boundaries. This looks a little more difficult, but it is again simple addition.
- How to work it: As an example, let's say we are given an address of 165.245.12.88/26
- Convert to decimal Using chart one, we see /26 sits on the 192 decimal line.
- Use decimal to get Netmask In chart 1, /26 is in the fourth column, so that means whatever our decimal is will go in the last octet. Octets before that will always be 255, and any after would be 0. So, in our case, the netmask will be 255.255.255.192
- Find your boundaries (Network and Broadcast addresses) Whichever octet of our netmask that we got from our decimal will tell us where to look for our network boundaries in chart two. In our example it is the 4th, 88. We also need to know how many addresses we have, so look at chart one in the last column for our CIDR (ours is 64). Take this info to chart 2, and find where our last octet falls on the 64 row. Ours is between 64 and 128, so our starting network address is 265.245.12.64. To find the broadcast, all we need to do is subtract 1 from the next number in our row. In our case, the next number is 128, so our broadcast is 127.
- Find the first and last usable IP address First usable? Add 1 to your network address. Last usable? Subtract 1 from your broadcast address.
Credit where credit is due: https://www.youtube.com/watch?v=ZxAwQB8TZsM