500
IPsec VPN key negotiation (IKE / IKEv2)
Internet Key Exchange (IPsec)
What is port 500?
UDP port 500 is used by IKE (Internet Key Exchange) to negotiate IPsec security associations and establish VPN tunnels. IPsec powers site-to-site VPNs (router-to-router) and the L2TP/IPsec and IKEv2 client protocols. It is used by Cisco, Juniper, pfSense, and the built-in VPN client on Windows. Together with UDP 4500 (NAT traversal), these are the two core ports required for IPsec VPN operation.
Quick checks
$
nc -zv example.com 500
$
nmap -p 500 example.com