Hi Adam, Thanks, i think i get most of it, i just want to make sure i have the math system down. The easy tricks like inverting or subtracting 255 i get, but i really want to know the logic behind it and get it down before continueing (although the more examples i see the more i feel i understand it). The bit about matching odd and even networks really threw me off at first. It seems the wildcard mask is more like a bitmask then anything else. And the odd/even thing i'm still not entirely sure. I.e. there is a lab somewhere where i think, 200.0.0.0 0.0.254.0 is used and 200.0.1.0 0.0.254.0
so same wildcard mask, different network and both match either odd or even. I understand if the lsb is set you always get an odd, and otherwise even, but what does having a different network value tip the scale for that? On Jun 1, 2011, at 11:47 PM, Adam Booth wrote: > Hi Alef, > > Which part is getting you confused? > > One of the most important things to be aware of is that a wildcard mask is > not just an inverted network mask. > > A rule regarding netmasks is that when looking at it as a binary sequence it > has to be a bunch of consecutive 1s and 0s, as soon as the first 0 appears, > the following digits must continue to be 0 > > For a wild card mask, the above concept does not apply, 1s and 0s can be in > any order. The binary sequence of the wild card mask has a meaning that a 0 > means, the value of this bit much match, where a 1 means, I don't care what > the source value is and is therefore an implied match. > > To give a simple example, say you are in OSPF and have an interface > 192.168.1.1/24 that you wish to enable. > > You could be most specific and do > > router ospf 1 > network 192.168.1.1 0.0.0.0 area 0 > > or more general and do > > router ospf 1 > network 192.168.1.0 0.0.0.255 area 0 > > I guess you could just go directly to the interface and do "ip ospf 1 area 0" > but that kind of sidelines the use case here, so lets ignore that for the > time being :) > > in both cases OSPF would be enabled for the interface, however if you wanted > to have the most direct control, specifying the complete address and using an > exact wildcard mask ensures that only that interface gets activated. For > example if I renumbered the interface on 192.168.1.1/24 to 192.168.1.111/24 > in the first case OSPF would be disabled for the interface and the later it > would not since the IP still matches the wildcard mask. > > There are also interesting things you can do with ACLs to match multiple > non-consecutive IP addresses by use of an interesting wildcard mask, these > rely on a base IP address and a wildcard string that relies on must > match/don't care values for the binary string. > > Is that of any help? > > Cheers, > Adam > > > On Thu, Jun 2, 2011 at 7:22 AM, Alef <[email protected]> wrote: > Does anyone have any recommendations for a good tutorial on wildcard masks? I > know the one on the ipexpert site, i'm afraid i need some more material to > digest if possible ;-) > i'm somewhere in between getting it and not getting it i think > thanks, > alef > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com
