Port 646: LDP
Port 646 is LDP, the Label Distribution Protocol that MPLS routers use to agree on the labels that steer traffic through a label-switched path. It is a core building block of carrier and large enterprise MPLS networks.
| Port number | 646 |
|---|---|
| Service | Label Distribution Protocol |
| Protocol | TCP |
| Category | Routing |
| Default encryption | No |
How do I check whether port 646 is open?
Port 646 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 646 | 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 646 | 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 ':646 ' | Lists the process bound to port 646 on Linux. On macOS use lsof -nP -iTCP:646 -sTCP:LISTEN, on Windows netstat -ano | findstr :646 |
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 646 used for?
Port 646 is LDP, the Label Distribution Protocol that MPLS routers use to agree on the labels that steer traffic through a label-switched path. It is a core building block of carrier and large enterprise MPLS networks.
Is port 646 TCP or UDP?
Port 646 (LDP) uses TCP.
Is port 646 secure?
Port 646 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 646 on my firewall?
Only if you specifically need LDP. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.