Subnetting Quick Reference Chart...
If you struggle with subnetting, or need something to help you become faster or more accurate, then this chart might be of use to you.
I keep a small copy of this at my desk and use it occasionally to check my work whenever it's done in my head.
Obviously there are some /'s and .'s that you use and know immediately right off the top of your head, but when the pressure is on (like some serious troubleshooting or when something is broke), this comes in handy to make sure your ACL's, mask's, etc.. are all correct
2 4 8 16 32 64 128 <-- # of Networks, if bit is used
128 64 32 16 8 4 2 1 <-- Bit Value in the Octet (and # of hosts if bit is used, but subtract 2)
.128 .192 .224 .240 .248 .252 .254 <-- Decimal Notation
/25 /26 /27 /28 /29 /30 /31 /32 <-- Slash Notation (if 4th octet)
/17 /18 /19 /20 /21 /22 /23 /24 <-- Slash Notation (if 3rd octect)
The Network address is always EVEN (or zero)
The first Host address is always ODD
The Broadcast address is always ODD
Not everybody needs something like this, and although I can subnet in my head, I find it much faster (and more accurate) to sometimes just look at my little chart and not have any doubt..
I've also used this to help teach those that are just beginning to learn subnetting and it has worked well.
I keep a small copy of this at my desk and use it occasionally to check my work whenever it's done in my head.
Obviously there are some /'s and .'s that you use and know immediately right off the top of your head, but when the pressure is on (like some serious troubleshooting or when something is broke), this comes in handy to make sure your ACL's, mask's, etc.. are all correct
2 4 8 16 32 64 128 <-- # of Networks, if bit is used
128 64 32 16 8 4 2 1 <-- Bit Value in the Octet (and # of hosts if bit is used, but subtract 2)
.128 .192 .224 .240 .248 .252 .254 <-- Decimal Notation
/25 /26 /27 /28 /29 /30 /31 /32 <-- Slash Notation (if 4th octet)
/17 /18 /19 /20 /21 /22 /23 /24 <-- Slash Notation (if 3rd octect)
The Network address is always EVEN (or zero)
The first Host address is always ODD
The Broadcast address is always ODD
Not everybody needs something like this, and although I can subnet in my head, I find it much faster (and more accurate) to sometimes just look at my little chart and not have any doubt..
I've also used this to help teach those that are just beginning to learn subnetting and it has worked well.