Subnetting

[dbasnett]

While looking for something else I stumbled upon this old thread, and decided to reply.

The most important point is that today's networks are classless (the C in CIDR), but most networking professionals understand that when you say Class A, B, or C, you mean /8, /16, or /24. There are certain instances that a default mask will be provided as a starting point based on the old classful rules. An example of that is when you configure the IP address on windows.

The best practice today is to make a statement such as, "My network is 192.168.10.0 /23".

Here is 192.168.10.0 /22 (NOTE 22) subnetted, which means that 192.168.8.0 /22 is the super-net of all of the following:

    Network           Net Broadcast     CIDR Mask              UsableHosts 
    192.168.8.0       192.168.9.255     23   255.255.254.0     510
    192.168.10.0      192.168.10.255    24   255.255.255.0     254
    192.168.11.0      192.168.11.127    25   255.255.255.128   126
    192.168.11.128    192.168.11.191    26   255.255.255.192   62 
    192.168.11.192    192.168.11.223    27   255.255.255.224   30 
    192.168.11.224    192.168.11.239    28   255.255.255.240   14 
    192.168.11.240    192.168.11.247    29   255.255.255.248   6  
    192.168.11.248    192.168.11.251    30   255.255.255.252   2  
    192.168.11.252    192.168.11.255    30   255.255.255.252   2

You should also note that 192.168.8.0 /22 is the base network.

Going back to the original, 192.168.10.0 /23...

The network number is 192.168.10.0. The IPv4 directed broadcast is 192.168.11.255. The usable hosts are
192.168.10.1 - 192.168.11.254. If you split it into 4 equal parts(subnet it) it looks like this:

    Network           Net Broadcast     CIDR Mask              UsableHosts 
    192.168.10.0      192.168.10.127    25   255.255.255.128   126
    192.168.10.128    192.168.10.255    25   255.255.255.128   126
    192.168.11.0      192.168.11.127    25   255.255.255.128   126
    192.168.11.128    192.168.11.255    25   255.255.255.128   126