Most users will not be exposed to SNMP (nor will they ever find port 161 open) unless some piece of their networking equipment has an active and open SNMP service port. If our port analysis ever shows that a router (for example) or other network device exposed to the Internet has its SNMP interface open you will want to arrange to disable and close that port immediately. Malicious hackers know that some consumer networking equipment has been shipped with exposed SNMP ports and with default access passwords. Therefore, it would not be at all unlikely that such a router or other equipment would be quickly discovered and exploited. Malicious hackers would find this amusing, but you would probably not.

SNMP RFC

The complete specification of the "Simple" (yeah, right) Network Management Protocol:

  http://www.ietf.org/rfc/rfc1157.txt

  http://www.faqs.org/rfcs/rfc1157.html