Direct answer and scope

The UDP port commonly used for IKE (Internet Key Exchange) traffic is UDP 500 for the initial IKE negotiation. In many real-world deployments, related traffic for NAT traversal can additionally use UDP 4500—so the “right” port depends on whether NAT traversal is in play.

Because this is a networking concept, exact port behavior can vary by configuration and IPsec/IKE implementation. The most reliable approach is to confirm what your specific setup expects (for example, in your gateway configuration and firewall rules) rather than assuming only one port will appear in every environment.

Simple model: how the ports fit together

Think of IKE as the protocol that establishes security parameters for IPsec. The initial negotiation typically runs on UDP 500.

When devices are behind NAT, there can be difficulty maintaining stable mappings for UDP flows. Many deployments therefore use NAT traversal, which relies on a different UDP port—commonly UDP 4500—so that NAT mappings remain consistent and the exchange can proceed.

This leads to a practical rule of thumb:

  • If you see only the initial IKE negotiation, UDP 500 is the port to expect.
  • If you also see traffic associated with NAT traversal, UDP 4500 may be present.

Key differences and limits

The key limitation is that “IKE traffic” is not always a single, one-port stream. Depending on how your IPsec/IKE configuration is done and whether NAT traversal is enabled, observers may see traffic on UDP 500, UDP 4500, or both.

Another limit is that not every environment labels captured flows as “IKE.” Firewalls, routers, and packet captures may show protocols in different ways, and some ports may be permitted even if a particular negotiation never occurs.

So while UDP 500 is the common answer for IKE, you should treat UDP 4500 as an important exception to check, especially if NAT is present between peers.

Practical checks you can do

  1. Verify your gateway/IKE configuration: Look for settings that enable NAT traversal or similar options, which often correspond to UDP 4500 usage.
  2. Check firewall rules: Confirm that outbound/inbound rules permit UDP 500 and (if NAT traversal is enabled or expected) UDP 4500 between the relevant peers.
  3. Inspect packet captures: When troubleshooting, filter for UDP 500 and UDP 4500 to see which port(s) are actually being used during negotiation.

If you only see UDP 500, that suggests NAT traversal may not be in use. If you see UDP 4500 as well, it indicates a deployment path that accounts for NAT.