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Definition: subnet mask tables


The following summarizes how networks, subnetworks and hosts are identified in the TCP/IP protocol. An IP address is first divided between networks and hosts. The host bits are further divided between subnets and hosts. See subnet mask and subnet.




Summary of Classes A, B & C
There are very few Class A networks, but each one can contain thousands of subnets and hosts (defined in 24 bits).



 CLASS B SUBNET MASKS (LARGE NETWORKS)

                 How 16
                 Bits Are  ---Maximum---
 Subnet Mask     Divided   Subnets/Hosts

 255.255.192.0      2/14       2  16,382
 255.255.224.0      3/13       6   8,190
 255.255.240.0      4/12      14   4,094
 255.255.248.0      5/11      30   2,046
 255.255.252.0      6/10      62   1,022
 255.255.254.0      7/9      126     510
 255.255.255.0      8/8      254     254
 255.255.255.128    9/7      510     126
 255.255.255.192   10/6    1,022      62
 255.255.255.224   11/5    2,046      30
 255.255.255.240   12/4    4,094      14
 255.255.255.248   13/3    8,190       6
 255.255.255.252   14/2   16,382       2



  CLASS C SUBNET MASKS (SMALL NETWORKS)

                 How 8
                 Bits Are  ---Maximum---
 Subnet Mask     Divided   Subnets/Hosts

 255.255.255.192   2/6         2     62
 255.255.224.224   3/5         6     30
 255.255.240.240   4/4        14     14
 255.255.248.248   5/3        30      6
 255.255.252.252   6/2        62      2













Class C Subnet Masks
The last 8 bits of the mask identify subnets and hosts. Network administrators have to determine how best to construct the network: more subnets and fewer hosts or more hosts and fewer subnets.