A network administrator’s tasks generally fall into the areas of designing and planning the network; setting up the network; maintaining and expanding it.

The first phase in the life cycle of a network involves creating its design, a task which entails making decisions about the type of network that best suits the needs of your organization.

Once the new network is designed, the second phase, which involves setting up and configuring the network, begins. This consists of installing the hardware that makes up the physical part of the network, and configuring the files or databases, hosts, routers, and network configuration servers.

The third phase of network administration consists of ongoing tasks that typically constitute most of what you are responsible for. They might include adding new host machines to the network, administering network security, administering network services, such as NFS™ services, name services, and electronic mail, and troubleshooting network problems.

The longer a network is in place and functioning properly, the more your organization might want to expand its features and services. Eventually, a single network will expand to the point where it can no longer operate efficiently. That is when it must enter the fourth phase of the network administration cycle: expansion.

Each task area corresponds to a phase in the continuing life cycle of a network. You might be responsible for all the phases, or you might ultimately specialize in a particular area, for example, network maintenance.

(https://docs.oracle.com/cd. Adaptado)