Subnetting Class B

[maskoff]

p258 to p261 Subnetting Class B address

practice examples 1B to 6B were good until

practice examples 7B

the book use the 4th octed to calculate the Valid Subnets
previouse examples use 3rd octed to calculate the Valid subnets.

practice example #6B uses 3rd octed (255)
255.255.255.128.
practice example #7B uses 4th octed (192)
255.255.255.192

Can someone explain why?

I must be missing something here..Thanks

[Fuzz]

It depends on which class of address you use as to which subnet mask octets are important. Class C addresses only have the fourth octet as important, but class B addresses have both third and fourth important. Only one octect will ever be 'interesting' though, and this is the one you use to calculate block sizes (interesting octects are neither 255 or 0).

[maskoff]

thanks Fuzz that will enlighten me more as I do more subnetting exercises.:):):)