SNMP
/ S-N-M-P /
Picture a building with hundreds of machines, and a facilities manager who needs to know each one's temperature, power draw, and whether its fan is spinning. They cannot walk to every machine, so each machine keeps a standard panel of dials the manager can read remotely. SNMP, the Simple Network Management Protocol, is exactly that scheme for networks: it lets a central manager ask thousands of routers, switches, servers, and printers "how are you doing?" in a uniform way, and read back the answers.
It has three parts. On each managed device runs a small program called an SNMP agent. The agent exposes a tree of named values called the MIB, the Management Information Base — things like "bytes received on interface 3," "CPU utilisation," or "interface up/down status," each addressed by a numeric object identifier (OID). A central network management station then sends SNMP get requests to read those values (and occasionally set requests to change a setting), polling devices on a schedule. The device can also push an unsolicited alert, called a trap, when something noteworthy happens, like a link going down — so the manager hears about a failure without waiting for the next poll.
Be honest about its age and security. SNMP version 1 and 2c authenticate only with a plaintext "community string" sent in the clear, which is barely a password — anyone who can see the traffic can read it, so these versions must never face the open Internet. SNMPv3 added real authentication and encryption and is what you should use. SNMP is also a polling model: ask, get an answer, ask again. Polling thousands of counters every few seconds does not scale gracefully, which is one reason modern systems are moving toward streaming telemetry, where devices continuously push their own data instead of waiting to be asked.
A monitoring server polls a switch every 30 seconds with an SNMP get for the OID that holds "octets in on port 5." Subtracting consecutive readings gives the bytes-per-30-seconds, which it plots as a bandwidth graph. When the switch loses a link it also fires a trap so the alert arrives immediately, not 30 seconds later.
An SNMP manager polls each device's agent for named MIB values and receives traps for urgent events — the classic network-management loop.
SNMPv1/v2c authenticate only with a cleartext community string — effectively no security — so never expose them to the Internet; use SNMPv3. Its poll-and-answer model also scales poorly, which is why streaming telemetry is increasingly preferred.