Port 631: IPP
Port 631 is the Internet Printing Protocol, the modern standard for sending print jobs and querying printer status over the network. CUPS, the print system used by Linux and macOS, runs its web interface and printing service on this port.
| Port number | 631 |
|---|---|
| Service | Internet Printing Protocol |
| Protocol | TCP/UDP |
| Category | Printing |
| Default encryption | No |
How do I check whether port 631 is open?
Port 631 carries TCP, so a connection either completes its handshake or it does not, which makes the check definitive.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | nc -vz example.com 631 | A TCP handshake either completes or it does not, so netcat gives a definitive answer in one line. |
| Same check on Windows | Test-NetConnection example.com -Port 631 | Built into PowerShell. TcpTestSucceeded in the output is the answer; telnet is not installed by default on modern Windows. |
| What is listening locally? | ss -tlnp | grep ':631 ' | Lists the process bound to port 631 on Linux. On macOS use lsof -nP -iTCP:631 -sTCP:LISTEN, on Windows netstat -ano | findstr :631 |
| Is it open on a remote host? | sudo nmap -sU -p 631 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':631 ' | Shows the UDP socket bound to port 631. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 631 used for?
Port 631 is the Internet Printing Protocol, the modern standard for sending print jobs and querying printer status over the network. CUPS, the print system used by Linux and macOS, runs its web interface and printing service on this port.
Is port 631 TCP or UDP?
Port 631 (IPP) uses TCP/UDP.
Is port 631 secure?
Port 631 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 631 on my firewall?
Only if you specifically need IPP. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.