StormWind.com Cisco Mentor Question Spotlight – Why Minus 2 in Host CalculationPublished by Anthony Sequeira | April 9, 2012
“Why do we need to subtract 2 when we are calculating the available host bits given a particular subnetting scheme? I remember that setting the host bits to all 1s indicates a broadcast for that subnet, but I cannot remember the other reason!”
Thanks so much for the awesome question. Yes, you are correct. You remembered one of the reasons why we must subtract a combination of bits settings. We subtract one for the case where the host bits are all set to 1s as you indicated. This is indeed for the broadcast on the subnet. But remember, we also subtract one for the case where all of the host bits are set to 0. This would indicate the subnet address itself! Think of it this way. If you live at 1201 Jones Street, the all 0s in the host bits would enable you to describe yourself as living on simply, Jones Street.
So the summarize (no pun intended), we subtract two when we are calculating the number of hosts to accommodate the all 1s and all 0s host bit combinations which are not valid for host addressing.
The Cisco Mentor Team