[ale] IP Calculations

[Joseph A Knapka]

Matt Shade wrote:
> 
> I'm not exactly sure what's being meant as "classless" and "classful", but
> basically, you have "default" subnet masks for Class A, B, C networks. Your
> default subnet masks are:
> 255.0.0.0        Class A networks - 0-127.x.x.x
> 255.255.0.0     Class B networks - 128-191.x.x.x
> 255.255.255.0 Class C networks - 192-223.x.x.x
> 
> These give you your basic Class networks. You can SUBnet these networks
> using non-default subnets.

My understanding (which could be wrong) is that you can
(now) subnet them any old way you want; network classes
have no meaning in the presence of CIDR. You can have a
192.168.* net with a 16-bit netmask, for example. That
wasn't always possible -- in the old days, if you were a
class A network your netmask was eight 1's and 24 0's,
no question about it -- but with CIDR==Classless InterDomain
Routing, anything goes.
 
> Subnet 255.255.255.4 is an odd subnet, but not incorrect. You basically need
> all the binary digits to be zeros on the right side, for example:
> 
> 11111111.11111111.11111111.11000000
> subnet mask of 255.255.255.192
> 
> 11111111.11111111.11111111.11111100
> subnet mask of 255.255.255.4

Hmm? 255.255.255.4 == 11111111.11111111.11111111.00000100b.
 
> 11111111.11111111.11111111.11110010
> incorrect, since all the numbers to the right of the last contiguous 1 is
> not a zero

Well, I've actually seen at least one network subnetted
that way. I just don't know if it was "correct" or
not, but it -did- work. Very confusing to add new
hosts, though. I have no idea what the net admin guy
was thinking.

Cheers,

-- Joe

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