An SNMP scanner is a program that sends SNMP requests to multiple IP addresses, trying different community strings and waiting for a reply. Unfortunately SNMP servers don’t respond to requests with invalid community strings and the underlying protocol does not reliably report closed ports. This means that ‘no response’ from the probed IP address can mean which of the following:

(Select up to 3)

A.
Invalid community string

B.
S-AUTH protocol is running on the SNMP server

C.
Machine unreachable

D.
SNMP server not running

Explanation:
http://en.wikipedia.org/wiki/Simple_Network_Management_Protocol