docs: Add comprehensive guide on understanding localhost URLs for local development

docs/understanding_localhost_urls.md

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+# Understanding Localhost URLs (like http://120.0.0.1)
+
+When you're working with web servers or network applications locally on your computer, you'll often encounter URLs that start with `http://127.0.0.1`. This document explains what these components mean.
+
+Let's break down an example URL structure often seen in local development: `http://127.0.0.1:PORT/PATH`
+
+## 1. `http://` - The Protocol
+
+*   **Stands for:** HyperText Transfer Protocol.
+*   **Purpose:** It's the foundational protocol for data communication on the World Wide Web. It defines how messages are formatted and transmitted, and how web servers and browsers should respond to various commands.
+*   **Communication Model:** HTTP works on a request-response model. A client (like your web browser or an application) sends an HTTP request to a server, and the server sends back an HTTP response (e.g., a web page, data, or an error message).
+*   **`https://` (HTTPS):** You might also see `https://`. The 'S' stands for Secure. HTTPS is a secure version of HTTP where the communication is encrypted, typically using SSL/TLS. For local development, `http://` is very common as the initial setup for security is often an extra step not immediately necessary.
+
+## 2. `127.0.0.1` - The IP Address (Loopback/Localhost)
+
+*   **Special IP Address:** `127.0.0.1` is a special-purpose IPv4 address.
+*   **Loopback Address:** It is known as the **loopback address**. Network requests sent to this address are "looped back" directly within your own computer. The data doesn't leave your machine to go onto your local network or the internet.
+*   **Localhost:** It is most commonly referred to as **localhost**. When a program on your computer accesses `127.0.0.1`, it is communicating with another program (or itself) running on the *same machine*.
+*   **Universal Standard:** This is a universally recognized standard. All computers using the TCP/IP protocol suite (which is virtually all modern networked computers) understand `127.0.0.1` to mean "this computer."
+*   **Development & Testing:** This IP address is extensively used by developers to run and test server applications (like web servers, database servers, or application backends like your Gradio server) directly on their own machines before deploying them to a public environment. It allows you to access the server as if it were remote, but without any actual network latency or external connectivity.
+
+## 3. `:PORT` - The Port Number (e.g., `:7860`)
+
+*   **Purpose:** A computer can run many different network applications or services simultaneously (e.g., a web server, an email server, a database). To distinguish between these services running on the same IP address (`127.0.0.1` in this case), **port numbers** are used.
+*   **Analogy:** Think of the IP address as the street address of an apartment building, and port numbers as the individual apartment numbers within that building.
+*   **Range:** Port numbers range from 0 to 65535.
+    *   Ports 0-1023 are "well-known ports" typically reserved for standard services (e.g., HTTP usually uses port 80, HTTPS uses port 443). Using these often requires administrative privileges.
+    *   Ports 1024-49151 are "registered ports" that can be registered by software vendors.
+    *   Ports 49152-65535 are "dynamic" or "private ports" that can be used by any application. Development servers often pick a port in this range or the registered range (like Gradio's default `7860`).
+*   **Example:** In `http://127.0.0.1:7860`, the `:7860` means the client should connect to the service running on port 7860 of the local machine.
+
+## 4. `/PATH` - The Resource Path (e.g., `/gradio_api/mcp/sse`)
+
+*   **Purpose:** Once connected to the server at the correct IP address and port, the path specifies the particular resource or service endpoint you want to access on that server.
+*   **Analogy:** If the IP and port get you to the correct "building" and "apartment (service)," the path is like asking for a specific "room" or "item" within that apartment.
+*   **Structure:** It's a hierarchical path, similar to file system paths.
+*   **Examples:**
+    *   In `http://127.0.0.1:7860`, if no path is specified (or just `/`), the server usually returns a default resource (like the main web page of the Gradio UI).
+    *   In `http://127.0.0.1:7860/gradio_api/mcp/sse`, the `/gradio_api/mcp/sse` part tells the server listening at `127.0.0.1:7860` that the request is specifically for the MCP service located at that path.
+
+## Summary
+
+So, when you see a URL like `http://127.0.0.1:7860/gradio_api/mcp/sse`:
+
+*   You are telling your client (browser or MCP client application) to:
+    1.  Use the **HTTP** protocol for communication.
+    2.  Connect to your **own computer** (localhost, `127.0.0.1`).
+    3.  Connect to the application that is listening on **port `7860`**.
+    4.  Request the specific resource or service located at the path **`/gradio_api/mcp/sse`** on that server.
+
+This setup is fundamental for local web development and testing network services.