Two reasons :
1. Legacy configuration portability, people learned a certain way and all versions of code understand a certain way. The best way to correct that issue it to accept either of them. 2. The inverse mask is indeed a pain in the neck but is technically correct. The subnet mask is used where the equipment cares to work with the network portion of the address (ignoring the host). The inverse mask is important where the equipment cares more about the host we are referring to (ignoring the network). It’s a bit of a cheat to allow for code used in routing to be used for ACL and firewall without modification to the code. For example, the same code piece that routes a network toward an Ethernet interface can be reused to route a host toward a null interface. Steven Naslund Chicago IL >Why do we still have network equipment, where half the configuration requires >netmask notation, the other half requires CIDR and to throw you off, they also >included inverse netmasks.
