SNMP monitors and manages network devices. v1/v2c use cleartext community strings (default 'public'). SNMPv3 adds authentication and encryption — the only secure version.
SNMPv1/v2c community strings are transmitted in cleartext — trivially sniffed. Default 'public' community string exposes device configuration. Always use SNMPv3 with auth+privacy mode. Block SNMP (UDP 161/162) from external access.