![class b subnet mask table class b subnet mask table](https://cdn.ttgtmedia.com/rms/onlineImages/networking-subnetting_desktop.png)
The rationale behind an inverse mask is that logical ANDing an inverse mask and an IP address gives the host portion of the address instead of the network/subnet portion (as a standard subnet mask would), which is of more interest to certain TCP/IP functions.
![class b subnet mask table class b subnet mask table](https://image3.slideserve.com/5568765/class-b-subnet-chart-l.jpg)
+ The inverse mask is primarily used in Cisco access control lists (ACLs). The +1 after each number indicates the extra subnet which can be reclaimed in this way. In this case you get one extra subnet per classful network, with the slight drawback that the network address of the network becomes the network address of the first subnet instead. However, on most newer routing and other networking equipment, you can specify that it use subnet zero (the command on a cisco is ip subnet-zero). The network mask is a 32-bit number with all ones for all network and subnet address portions, and all zeros for the host field. This means that in classful addressing, the first and last subnets within a network are unusable, since all subnet bits would be zeroes or ones. * Generally, host bits, subnet bits, and network bits cannot consist of all ones or all zeroes without conveying special meaning to the address (network address, broadcast address, etc). Invalid mask since it leaves no host bits Any Class A network has a total of 7 bits for the Network ID (bit 8 is.
![class b subnet mask table class b subnet mask table](http://image.slidesharecdn.com/vlsmsubnettingchart-121017211529-phpapp02/95/vlsm-subnetting-chart-1-638.jpg)
In class B or larger networks, CIDR, or subnet-zero enabled networks only The Class A IP Addresses were designed for large networks, Class B for medium. In Class A networks, CIDR, or subnet-zero enabled networks only