|  | 10.7 | Creating a Subnet
| 10.7.7 | Host/subnet schemes
One of the decisions that you must make whenever you create subnets is to determine the optimal number of subnets and hosts (Note: The number of subnets required in turn determines the number of hosts available. For example, if you borrow 3 bits with a Class C network, only 5 bits remain for hosts).
| Lab Activity
| This lab focuses on a Class C network with three subnets and using a Custom Subnet Mask. It will help you develop a better understanding of IP subnet masks.
You have already learned that you cannot use the first and last subnet. You also cannot use the first and last address within each subnet - one is the broadcast address of that subnet, and the other is part of the network address. When you create subnets, you lose quite a few potential addresses. For this reason, network administrators must pay close attention to the percentage of addresses that they lose by creating subnets. Example: If you borrow 2 bits with a Class C network, you create 4 subnets, each with 64 hosts. Only 2 of the subnets are usable and only 62 hosts are usable per subnet, leaving 124 usable hosts out of 254 that were possible before you chose to use subnets. This means you are losing 51% of your addresses. Imagine, this time, that you borrow 3 bits. You now have 8 subnets, of which only 6 are usable, with 30 usable hosts per subnet. This gives you a total of 180 usable hosts, down from 254, but now you are losing only 29% of your addresses. Whenever you create subnets, you need to take into consideration future network growth and the percentage of addresses that you would lose by creating subnets.
| Web Links
Introduction to the Internet Protocols