40,136,786 Answers on Port 500 and 303,376 on Port 4500: Reading the IKE Surface

A ZoomEye query for UDP port 500 returns 40,136,786 results. The same index, asked for port 4500, returns 303,376.
Both ports belong to IKE, the protocol that negotiates IPsec security associations, and the difference between them is about 132 to one. That gap is not noise. It reflects a specific operational property of how IPsec endpoints are deployed, and it has a practical consequence for anyone using exposure data to decide what to look at.

Context and method

Both queries were run on 27 September 2026 against the global ZoomEye index, counting hosts.
Port 500 carries the initial IKE negotiation. Port 4500 carries IKE when a NAT device is in the path, and it also carries the encapsulated ESP traffic that results when NAT traversal is in use. Standard IPsec endpoints listen on both, because they cannot know in advance whether a peer will be behind NAT.

Analysis

A result set of over 40 million on port 500 has to be read with care before it is read as an IPsec census. UDP 500 is a conventional port, and it is used for ISAKMP, but it also carries traffic from other protocols that chose the same number, and NAT devices and middleboxes may answer on it independent of any host behind them. The figure is a measurement of what responds, not a count of IPsec gateways.
The 303,376 on port 4500 is a different kind of number. NAT traversal is only used when needed, so this set is closer to the population of endpoints that have been configured for the reality of a NAT device in the path. That makes it a more specific signal, and a much smaller one.
The two figures together support a reasonable inference about placement. A count this large on 500, alongside a much smaller count on 4500, is consistent with a very large population of endpoints that are reachable at the negotiation layer while a much smaller proportion sit in NAT-traversed deployments. It is also consistent with the simpler explanation that port 500 is shared with more non-IKE services than 4500 is. Both readings point the same way for the purpose of prioritisation, which is that a port-500 count is a screening input rather than a finding.
What IKE exposes at the negotiation layer is worth stating precisely, because it is often described loosely. The protocol exchanges vendor identification, supported encryption and hash proposals, Diffie-Hellman group preferences and, in main mode, authentication material. That exchange is visible to anyone who can send a packet. The consequence is fingerprinting: the responder identifies its implementation family, which lets an observer narrow down which product and which firmware generation is answering, without authenticating.
The more consequential configuration issue is authentication method. Where IPsec peers authenticate with a pre-shared key, a weak or default key is a single secret protecting every peer in the group, and it is not rotated by certificate expiry because there is no certificate. Where peers authenticate with certificates, the failure mode shifts to validation: whether the endpoint checks the trust chain properly, whether it checks revocation, and whether it accepts a certificate presented for a different identity.

Implications

For the operator, the useful action is not to count. It is to confirm the authentication method on every IPsec peer, confirm the strength and uniqueness of a pre-shared key where one is used, and confirm that certificate validation is enforced rather than advisory. Where aggressive mode is in use, understand that it transmits identity information and is worth replacing with main mode if the peer population allows it.
For exposure assessment, the comparison suggests a filter that is more useful than either raw count. A host that answers on 4500 is more likely to be a genuine IPsec endpoint than one that answers only on 500, because fewer unrelated services arrive on that port, so 4500 is a reasonable starting filter when the goal is to find real VPN endpoints. A host that answers on 500 alone is a candidate that needs confirmation before it is treated as one.

Limitations

Both figures are host counts from a single index on a single day and depend on what is routable and how services respond. This article does not claim that a host counted on 500 or 4500 is an IPsec endpoint, does not claim that any endpoint is misconfigured, and does not claim that the 4500 group is a subset of the 500 group. The relationship between the two numbers is described as suggestive of placement, not as a derivation.

References

• ZoomEye, port="500" and port="4500", queried 27 September 2026: https://www.zoomeye.org/