|  | 1.3 | TCP/IP Addressing
| 1.3.2 | Subnetworks
Subnets improve the efficiency of network addressing. Adding subnets does not change how the outside world sees the network, but within the organization, there is additional structure. In Figure , the network 172.16.0.0 is subdivided into four subnets: 172.16.1.0, 172.16.2.0, 172.16.3.0, and 172.16.4.0. Routers determine the destination network by using the subnet address, which limits the amount of traffic on the other network segments. From an addressing standpoint, subnets are an extension of a network number. Network administrators determine the size of subnets based on the expansion needs of their organizations. Network devices use subnet masks to identify which part of the address is for the network and which part represents host addressing. Example of Class C subnetting. In Figure , the network has been assigned the Class C address 201.222.5.0. Assuming that 20 subnets are needed, with a maximum of 5 hosts per subnet, you need to subdivide the last octet into a subnet and a host, and then determine what the subnet mask will be. You need to select a subnet field size that yields enough subnetworks. In this example, selecting 5-bits gives you 20 subnets. In the example, the subnet addresses are all multiples of 8 - 201.222.5.16; 201.222.5.32; and 201.222.5.48. The remaining bits in the last octet are reserved for the host field. The 3 bits in the example are enough for the required five hosts per subnet (actually, giving you host numbers 1 - 6). The final host addresses are a combination of the network/subnet segment's starting address plus each host's value. The hosts on the 201.222.5.16 subnet would be addressed as 201.222.5.17, 201.222.5.18, 201.222.5.19, and so forth. A host number of 0 is reserved for the wire (or subnet) address, and a host value of all 1s is reserved because it selects all hosts-that is, it is a broadcast. A table used for the subnet planning example is on the following page. Also, a routing sample shows the combining of an arriving IP address with a subnet mask to derive the subnet address (also called the subnet number). The extracted subnet address should be typical of the subnets generated during this planning exercise. Example of Class B subnetwork planning In Figure , a Class B network is subnetted to provide up to 254 subnets and 254 useable host addresses. Example of Class C subnetwork planning In Figure , a Class C network is subnetted to provide 6 host addresses and 30 useable subnets.