Port 500 supports IKE / ISAKMP and isakmp (TCP reference coverage) across distinct transports. Review each protocol before writing firewall or monitoring policy.
Negotiates security associations and keys for IPsec VPNs.
Site-to-site and remote-access IPsec tunnel establishment.
Bidirectional
Use strong IKE versions, algorithms, identities, and pre-shared secrets or certificates; pair with UDP 4500 for NAT traversal.
isakmp (TCP reference coverage) is documented as a network service convention for port 500 over TCP.
Use this reference entry when identifying or validating systems documented to use 500/TCP.
Varies
Confirm the listening process and current vendor documentation before permitting traffic; a listed convention does not prove the active service or its security posture.