SNMP stands for Simple Network Management Protocol, but it is actually more than just a protocol—it’s a way of naming data, a grammar for describing data, and a protocol for exchanging data over a network.
These components are called the MIB (the naming scheme), SMI (the grammar) and the SNMP protocol. The RFCs that describe SNMP include a common set of information useful in managing networked devices. This is called called MIB-2
The managed devices (routers, switches, web hosts, desktop computers, network printers, and so on) each run a server process (daemon) called an SNMP agent. This agent listens on a particular UDP port (usually, but not always, port 161) for read (GET) and write (SET) commands. It’s the agent’s responsibility to fetch the requested data and return it. This agent, as well as other monitoring software on the device, may also send spontaneous notifications called traps or informs to one or more configured target systems.
Management applications (managers) poll the agents for information.
They may keep historical information, generate reports, or create graphical maps of the devices in your network. Some management applications are just simple command-line tools (like the snmpwalk, snmpget, and snmpset commands supplied with Net-SNMP).