Port 161: SNMP
Port 161 is SNMP, used by monitoring tools to read status from routers, printers and other devices.
What runs on port 161
SNMP agents listen on UDP 161 and answer queries about interfaces, traffic, toner levels, temperatures and more. Versions 1 and 2c protect access only with a "community string", often left at the default "public". Version 3 adds real authentication and encryption.
On a home network
Network printers very often run SNMP on 161, and so do many NAS boxes, managed switches, business routers and UPS units. Consumer routers sometimes offer it but leave it off.
If port 161 is reachable from the internet
SNMP should not be reachable from the internet. With the default "public" community, anyone can read detailed information about your device and network, and a writable community can change settings. SNMP v2c has also been abused for amplification DDoS because a small query returns a large answer.
How to check whether it is open on your network
Check whether SNMP is enabled in your router, NAS and printer settings, and change the community string from "public" if you use it. An outside scan of UDP 161 shows whether it is exposed.
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