Internet Key Exchange (IKE) versions 1 and 2 are designed to coexist on the same network devices. Both versions use UDP port 500 for the initial key exchange negotiation and UDP port 4500 for NAT traversal (NAT-T). The IKE header format includes a version number, which allows a responding device to identify the IKE version used by the initiator and reply using the same version. This shared port usage is a fundamental aspect of their interoperability.

A. Asymmetric authentication (where each peer can use a different authentication method) is a feature of IKEv2, but not IKEv1.

C. IKEv1 and IKEv2 both operate over UDP ports, not TCP ports, for their primary negotiations.

D. IKEv1 has a distinct two-phase structure (Phase 1 and Phase 2). IKEv2 streamlines this into a single initial exchange, so they do not share this concept.

1. FortiOS 7.6 IPsec VPN Administration Guide. (2024). "IKE versions" section, p. 21. "Both IKEv1 and IKEv2 use UDP port 500 as the source and destination port for IKE negotiations."

2. RFC 7296: Internet Key Exchange Protocol Version 2 (IKEv2). (October 2014). Section 1.2, "The IKEv2 protocol runs over UDP, using port 500 for initial packets... These are the same ports that IKEv1 uses."