Port 4500 — UDP
UDP port 4500 is used by IPsec NAT Traversal when a VPN path includes address translation. IKE negotiation commonly starts on UDP 500 before NAT-T uses UDP 4500. Check IKE, NAT detection and the transition to 4500 during diagnosis.
What is port 4500 used for?
When NAT is detected between peers, NAT-T can carry IKE and UDP-encapsulated ESP on UDP 4500. Seeing traffic on 500 without the expected traffic on 4500 can help identify a NAT-related problem.
Transport protocol
UDP
Security precautions
Often required alongside UDP 500. Restrict access to expected VPN peers where possible.
Should we open this port in the firewall?
An Internet-facing VPN gateway may legitimately require UDP 4500. Restrict known peers, use appropriate IKE/IPsec settings and monitor failures. Do not allow NAT-T to internal devices that have no VPN role.
How do I check this port?
Capture traffic with a filter such as udp port 500 or udp port 4500 and inspect IKE logs on both peers. Check NAT detection, translation state and session timeouts. Test-NetConnection -Port tests TCP and cannot validate UDP 4500.
- Confirm the initial IKE exchange on UDP 500.
- Verify the expected NAT-T transition to UDP 4500.
- Check NAT rules and session timeouts along the path.
- Then compare IPsec proposals, identities and selectors if the tunnel still fails.
FAQ — port 4500 / IPsec NAT-T
Why does IPsec use port 4500?
When NAT is detected, NAT-T encapsulates IPsec traffic in UDP 4500 to traverse address-translation devices.
Is port 500 still needed with port 4500?
In many deployments, yes: IKE starts on UDP 500 before switching to UDP 4500 after NAT detection.
What should I check if 500 works but 4500 does not?
Inspect NAT, UDP 4500 firewall rules, session timeouts and IKE logs on both peers.