Stupid Question maybe?

Tue Dec 18 22:00:21 UTC 2018

I see it more used in terms of firewall operations on what are normally network routing devices.  I suppose someone with Cisco IOS architecture inside knowledge could tell us why they use that notation with ACLs primarily.  

 I have never seen a computer want or accept an inverse mask so it is irrelevant to ARP.  The question with ARP is "are we on the same network".

The naming of inverse net mask is really tragic.  It should be called net mask and host mask because that is what they really are.  In a net mask the 1s denote the network portion, in the host mask (nee inverse netmask) the 1s denote the host portion.  That's all there is too it.

The inverse mask could be used to figure out whether to ARP or not.  You just have to decide if the 1s or 0s mean that something is significant or not significant to your calculation.  Using the inverse mask I could decide to dump the portion = 1.  Using the network mask I can dump the portion = 0.  Nothing states how you have to use the information.

Steve

>Hi Steve,
>
>That's like saying the inverse mask is technically correct when the computer wants to decide whether to arp for the next hop. No sale man.
>
>A AND NETMASK ?= B AND NETMASK
>
>is exactly the same operation as
>
>A OR inverse NETMASK ?= B OR inverse NETMASK
>
>While A AND inverse NETMASK ?= B AND inverse NETMASK *never* yields useful knowledge.
>
>No sale.
>
>Regards,
>Bill Herrin