First part is for a subnet mask that falls in the fourth octet. There are 256 possible numbers since zero is part of the set (0 -255). Subtract the subnet mask from 256 and the resulting number is the number of addresses per subnet (which includes the subnet identifier and the subnet broadcast address). 256 - 240 = 16. So all multiples of 16 are valid subnet identifiers with a /28. Another way to view it is to look at the bit position of the subnet mask 128, 192, 224, 240 248, 252, 254, 255. The 240 corresponds to the least significant bit of the higher nibble. It has a value of 16. Even easier if we count the bits from right to left as 0 to 7. Then the bit position is the power of 2. 2 to the 4th is 16. The subnet mask is also the last possible subnet - assuming classless behavior. If the subnet mask falls in the third octet take the difference between the subnet mask and 256 and multiply that value by 256 (the sum of all hosts in the fourth octet). Then subtract 2 for the subnet identifier and broadcast addresses. 255.255.248.0 256 - 248 = 8, 8 x 256 = 2048, 2048 - 2 = 2046 hosts per subnet. How many subnets in the above example? Count the network bits from left to right 1 to 8. 248 is the fifth bit. 2 to the 5th is 32. A classful view would have us discard the all zeros and all ones subnets so 32 - 2 = 30 subnets. A subnet mask that falls in the second octet would calculate the number of host addresses by (256 - mask) = result x 256 x 256 > -----Original Message----- > From: Priscilla Oppenheimer [mailto:[EMAIL PROTECTED]] > Sent: Thursday, August 09, 2001 12:16 PM > To: [EMAIL PROTECTED] > Subject: Re: Why Should the Binary Math Method Be Used to Subnet > [7:15488] > > > I do use decimal. ;-) Just like someone else said, I do make > a list such > as: 128, 192, 224, 240, 248, 252, 254, 255. > > I don't get the tricks I've seen people do where they > subtract from 256 (or > something like that. Since I don't get them, I can't remember them!) > > Priscilla > > > At 11:18 AM 8/9/01, Debbie Becker wrote: > >Oh, surely not brain damaged, Priscilla! Perhaps decimally > challenged? > > > > > >Deb > > > >""Priscilla Oppenheimer"" wrote in message > >[EMAIL PROTECTED]">news:[EMAIL PROTECTED]... > >| Well, I have to admit to never really understanding the > shortcuts that I > >| have seen people use. ;-) I guess I'm brain damaged, but I > have to do it > >in > >| binary. > >| > >| As far as tools are concerned, I totally agree with the > person who pointed > >| out that in design meetings, you often don't have any tool > handy except > >| your head. Sure, you may have an IP subnet calculator on > your Palm, but > >the > >| client is going to look at you askance as you bury your head in the > >| itty-bitty screen, trying to use that silly stylus thing > or Graffiti, > >| giving them no eye contact the whole time you're puttering > with the tool. > >| Use your head and look professional and like you're worth > the big bucks > >| that they are paying you. > >| > >| Priscilla > >| > >| At 03:52 PM 8/8/01, Debbie Becker wrote: > >| >I use decimal shortcuts most of the time as well -- but > when I run across > >| >something confusing, I'll go back to binary -- it always > shows me the way > >FAQ, list archives, and subscription info: > >http://www.groupstudy.com/list/cisco.html > >Report misconduct and Nondisclosure violations to > [EMAIL PROTECTED] > > > ________________________ > > Priscilla Oppenheimer > http://www.priscilla.com > Report misconduct > and Nondisclosure violations to [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=15523&t=15523 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
