Why subnetting?

By creating smaller IP networks (instead of having one large network), we can obtain better security, smaller collision and broadcast domains, and greater administrative control of each network. Think of a network like streets in a city. Each house on this network is known by the street and by the address. Think of the addresses on the houses as the hardware addresses of a host. For IP to communicate with a host, the IP address must be known, and the router connected to the network on which this host is located must also know the hardware address of the house.What if a city didn’t have many blocks but just one long street? The mailman would go crazy trying to get the mail delivered to each house correctly because he would have to know the address of every house. It’s the same scenario with IP. By creating smaller networks, we can more effectively get data to each host.Subnetting a Class C networkSo you understand why you want to subnet, but how do you do it? Your goal is to look at an IP address and subnet mask of a host and then determine three things quickly:

  1. The subnet the host is located in
  2. The broadcast address of the subnet
  3. The valid host range of the subnet used to configure hosts

Once the subnet is determined, the broadcast address must be found. Why? Because these are not valid host addresses and cannot be assigned to host configurations. Also, by determining the subnet and broadcast addresses, we can easily determine the host addresses because the valid host range is always the numbers between the subnet address and the broadcast address.

If we use the default subnet mask with a Class C network address, then we already know that three bytes are used to define the network and only one byte is used to define the hosts on each network.