# Core NATS Core NATS is the foundational functionality in a NATS system. It operates on a publish-subscribe model using subject/topic-based addressing. This model offers two significant advantages: location independence and a default many-to-many (M:N) communication pattern. These fundamental concepts enable powerful and innovative solutions for common development patterns, such as microservices, without requiring additional technologies like load balancers, API gateways, or DNS configuration. NATS systems can be enhanced with [JetStream](/nats-concepts/jetstream.md), which adds persistence capabilities. While Core NATS provides best-effort, at-most-once message delivery, JetStream introduces at-least-once and exactly-once semantics. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://docs.nats.io/nats-concepts/core-nats.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.