It's not the Nagios port but a SNMP port.
en.wikipedia.org/wiki/Simple_Net ... t_Protocol
so this line will tell you enough:
Typically, SNMP uses UDP ports 161 for the agent and 162 for the manager. The Manager may send Requests from any available ports (source port) to port 161 in the agent (destination port). The agent response will be given back to the source port. The Manager will receive traps on port 162. The agent may generate traps from any available port.