Appendix C: Subnet Masks
(Page 2 of 2)
Custom Subnet masks
Not all subnets are made up of 255 and zeros. The Class A subnet shown above allows 16,777,216 unique devices (hosts). That is quite a bit of network traffic. To keep things flowing more smoothly an network can be divided into separate physical areas, and traffic can be divided by group or geography. For example finance and engineering. To break this single Class A address into two physical areas, the subnet 255.192.0.00 would be created. One for engineering, the other for finance.
The subnet would look as shown in Figure 63.
The 192(11000000) creates two custom subnets (at the network layer)
In this case IP host addresses were borrowed to create separate physical spaces. This reduces the number of available host addresses by 2 bits. Going with 255.224.0.0 creates three physical subnets, and further reduces the number of hosts. Perhaps you may wish to think of IP addresses as a pie. You can create more slices and serve more individual pieces if you cut more and serve smaller pieces. However, 32 bits is 32 bits. Just like the pie, it all is determined in how you slice it.
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