Is there any tutorial on how to calculate netmask for private IP address?.

Is there any tutorial on how to calculate netmask for private IP
address (for example class B addr: 172.16.xx.xx , netmask 255.255.0.0,
why can't it be 255.255.255.0?). Thanks.

 

:Is there any tutorial on how to calculate netmask for private IP
:address (for example class B addr: 172.16.xx.xx , netmask 255.255.0.0,

The rules are the same as for any other IP address, namely
that one should look at the first octet. 0-127 as a first octet
is "Class A", 128-191 is "Class B", 192-222 is "Class C",
223 and 224 are reserved for multicast, and everything above is
reserved or special purpose.


What it means for an IP to fall into Class A, B, or C varies.

For older equipment, or for some of the more obscure syntax
variations in newer equipment, or if one is using the RIP1
routing protocol, then Class A implies a netmask of 255.0.0.0,
Class B implies a netmask of 255.255.0.0, Class C implies a
netmask of 255.255.255.0, and the equipment or protocol might
not allow those netmasks to be overridden.

The Cisco PIX is an example of equipment will use the Class A/B/C
netmasks if you leave out an optional netmask specification... which
can lead to some nasty surprises as the optional netmasks usually
occur in contexts where the user would think that the PIX should *know*
what the proper netmask should be, based upon the other commands.

For newer equipment, what it means for an IP to fall into
Class A, B, or C, is "nothing" (except perhaps in non-extendable
backwards compatability such as RIP1.) Newer equipment does not
assume any netmask. Except, that is, for consumer home routers and
firewalls, which are often hard coded to force a netmask of 255.255.255.0
on internal addresses.


:why can't it be 255.255.255.0?). Thanks.

172 falls into the range 128 to 191, so it is Class B, 255.255.0.0.

Why can't it be 255.255.255.0? Because that would require rewriting
decades of technical specifications and attempt to force equipment
that literally might have been designed 20 years ago to do something
that it was not designed to do. Unless someone can find several
billion barrels of oils that would become accessible if
172.16.*.* were redeclared to have a netmask of 255.255.255.0,
It Ain't Gonna Happen.

The good news is that most of the time it doesn't matter anymore.
Any modern (non-consumer) equipment will allow you to use
a netmask of 255.255.255.0 with 172.16.x.x if that's what you
want to do.

You have to know the old Class A/B/C system for tests (because you
might have to work with old equipment sometime), and you need to know
A/B/C in case you have reason to use RIP1, but if you want to
use a netmask of (say) 255.255.192.0 with an IP that starts with
172.16, then probably your biggest obstacle would be in documenting
why you happened to choose that particular mask; modern equipment
with modern protocols will handle that netmask without problem.

 

Thanks Walter, appreciated, nice one.