IPv4 Netmask reference

A netmask is a 32-bit mask used to divide an IP address into subnets and specify the networks available hosts. In a netmask, two bits are always automatically assigned. For example, in 255.255.225.0, "0" is the assigned network address; and in 255.255.255.255, "255" is the assigned broadcast address. The 0 and 255 are always assigned and cannot be used.

There are plenty of these netmask references out there, but I prefer my own: hence this Tech Tip. We've never seen anybody use a network larger than a /4 (256M hosts), so we've truncated the table at that point

 

What is cisco notation,,,?

When creating ACLs for Cisco routers, one specifies networks using a base IP address and what looks like an inverted netmask: rather than set a one bit for each part of the address that's a network, they set a one bit for each part that's a host.