In an SNMP configuration, a system known as a manager acts as the central communication point for all the SNMP-enabled devices on the network. On each device that is to be managed and monitored via SNMP, software called an SNMP agent is set up and configured with the IP address of the manager. Depending on the configuration, the SNMP manager is then capable of communicating with and retrieving information from the devices running the SNMP agent software. In addition, the agent is able to communicate the occurrence of certain events to the SNMP manager as they happen. These messages are known as traps.
An important part of SNMP is an SNMP management system, which is a computer running a special piece of software called a Network Management System (NMS). These software applications can be free, or they can cost thousands of dollars. The difference between the free applications and those that cost a great deal of money normally boils down to functionality and support. All NMS systems, regardless of cost, offer the same basic functionality. Today, most NMS applications use graphical maps of the network to locate a device and then query it. The queries are built in to the application and are triggered by a point and click. You can actually issue SNMP requests from a command-line utility, but with so many tools available, it is simply not necessary.
An SNMP agent can be any device capable of running a small software component that facilitates communication with an SNMP manager. SNMP agent functionality is supported by almost any device designed to be connected to a network.