Port 4500: IPsec NAT-T
Port 4500 carries IPsec traffic once IKE on port 500 detects that a NAT device sits between the two VPN endpoints. Wrapping the IPsec packets in UDP on this port lets them pass through routers that would otherwise break the tunnel.
| Port number | 4500 |
|---|---|
| Service | IPsec NAT Traversal |
| Protocol | UDP |
| Category | Routing |
| Default encryption | No |
How do I check whether port 4500 is open?
Port 4500 is UDP, which changes the answer: there is no handshake to complete, so nothing outside the host can prove the port is open. Asking the host itself is the reliable route.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | sudo nmap -sU -p 4500 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 ':4500 ' | Shows the UDP socket bound to port 4500. 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 4500 used for?
Port 4500 carries IPsec traffic once IKE on port 500 detects that a NAT device sits between the two VPN endpoints. Wrapping the IPsec packets in UDP on this port lets them pass through routers that would otherwise break the tunnel.
Is port 4500 TCP or UDP?
Port 4500 (IPsec NAT-T) uses UDP.
Is port 4500 secure?
Port 4500 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 4500 on my firewall?
Only if you specifically need IPsec NAT-T. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.