SNMPv1: RFC 1157
Basic functionality, supported by all vendors
SNMPv2: RFC 1905, 1906, 1907
Some useful additional features; supported by many vendors
SNMPv3: RFC 1905, 1906, 1907, 2571, 2572, 2573, 2574, 2575.
Still a proposed standard
Adds strong authentication
Supported by Net SNMP and some Cisco products