The Simple Network Management Protocol (SNMP) monitors network devices. It uses UDP port 161 and traps over UDP port 162. A trap is automatically sent to the client without request.
A Management Information Base (MIB) is a text file containing object identifiers and other information, written in Abstract Syntax Notation One format.
Version 1 has no built in authentication, version 2 is community based, and version 3 includes encryption and authentication.
Enumeration
Tools like snmpwalk, onesixtyone, and braa can be used to brute-force the community strings/passwords. A common community string is public.
snmpwalk -v2c -c <COMMUNITY_STRING> <IP>
onesixtyone -c
braa <COMMUNITY_STRING>@<IP>:.1.3.6.*