Using the Network Subnetting Tables

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Using the Network Subnetting Tables

As mentioned earlier, after you understand how to use the previous manual calculations for subnetting a network, you may want to use the tables provided to avoid the lengthy calculations.

Tables 4.9, 4.10, and 4.11 show the number of subnets that are used with a given subnet mask for each of the Class A, B, or C addressing types.

In the preceding tables, the Additional Bits Required is the number of higher-order bits required to be added to the default subnet mask to achieve the required number of subnets and hosts per subnet. For convenience, the resulting subnet mask is shown in decimal notation rather than in binary.

As an example, suppose you are assigned a Class B network ID of 168.20 that must be subdivided into 3 subnets with a maximum of 500 hosts on any given subnet. Adding three bits to the subnet mask allows for 6 subnets with 8,190 hosts on each subnet. However, this subnet mask does not allow for much growth in the number of subnets, while allowing for more than ample growth in the number of hosts on each subnet. On the other extreme, adding 7 bits to the subnet allows 126 subnets with only 510 hosts on each subnet. This subnet mask allows for a great deal of growth in the number of subnets but very little growth in the number of hosts on each subnet. A more appropriate subnet mask is somewhere in the middle. A subnet mask with 4 additional bits allows 14 subnets with 4,094 hosts on each subnet. A subnet mask with 5 additional bits allows 30 subnets with 2,046 hosts on each subnet. Either of these choices is good. You can lean toward one or the other depending on whether you anticipate greater fragmentation on your network in the future, thus requiring more subnets, or greater growth on existing network segments, thus requiring more hosts on each subnet.

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