Given the growing demand for a registry infrastructure for trusted discovery, feature negotiation, and identity assurance of autonomous AI agents, we analyze five leading approaches: (1) MCP Registry (centralized mcp.json descriptor publishing), (2) A2A Agent Cards (decentralized self-describing JSON feature manifests), (3) AGNTCY Agent Directory Service (IPFS Kademlia DHT content routing extension for semantic taxonomy-based content discovery, OCI artifact storage, and Sigstore-based integrity), (4) Microsoft Entra Agent ID (enterprise SaaS directory with policy and zero-trust integration), and (5) NANDA Index AgentFacts (cryptographically verifiable and privacy-preserving attestation model). We present architectural tradeoffs between centralized control, enterprise governance, and decentralized resilience using four evaluation dimensions: security, authentication, scalability, and maintainability. We conclude with design recommendations for a new Internet of AI Agents that require verifiable identity, adaptive search flows, and interoperable functional semantics.