Valid Subnets

[1007E1337]

hey folks, I am taking an intro to networking course and i have a question

if you have 512 subnets and you are classless running class B-

your mask would be 255.255.255.128

if you start at

172.16.0.0

your last subnet would be 172.16.255.128

if your incrimenting by 128 ( being your least significant bit ) - would your last subnet being 172.16.255.128-172.16.255.255 (is this valid and it knocks off one of your hosts -OR- does it want to use the .256 and becomes invalid?)

thanks in advance

[Majed Saeed]

hi john ,

see .... you have already defined the ip subnet & subnet mask ...

255.255.255.128 is = /25

I means you have only two subnnets

first pool start x.x.x.1 till x.x.x.126  (127 will be brdcst )

second pool start x.x.x.129 till x.x.x.254 (255 will be brdcst ) and note the ip x.x.x.128 in network id for second subnet .

regards,

majed

[1007E1337]

I actually was looking for an example where you had 512 subnets on class B - if the 512th subnet was valid or not

[Richard Burts]

John

Yes the 512th subnet is valid.

Many people get confused about how this works. And part of the confusion is that they take the principle that you can not use the first and last address in the subnet and they apply it to subnets (can you use the first and last subnet).

In the early days of subnetting they did apply the restriction about not using the first subnet (frequently referred to as subnet zero) and there was a rule (for a while) that you could not use subnet zero. Many people assumed that the principle would also cover the last subnet. However that was not true. And since the rule of do not use subnet zero has been deprecated for a long time, it is now legitimate to use both the first subnet of the major network and the last subnet of the major network. so a subnet of 172.16.255.128-172.16.255.255 is perfectly ok.

HTH

Rick