Glossary

Localhost

Definition: Localhost is the standard hostname referring to the current computer, resolving to the loopback IP address 127.0.0.1, used to access local development servers.

Localhost is the default hostname used to refer to the computer you are currently using. It resolves to the IPv4 loopback address 127.0.0.1 (or ::1 in IPv6). Network traffic sent to localhost never leaves your machine — it is handled entirely by the operating system's loopback interface.

How Developers Use Localhost

  • Running a local development web server: http://localhost:3000
  • Connecting to a local database: localhost:3306 (MySQL)
  • Testing API endpoints without deploying to a remote server.
  • Debugging applications in isolation from production systems.

Common Localhost Ports in Development

  • localhost:3000 — Node.js / React (Create React App)
  • localhost:4200 — Angular CLI
  • localhost:5000 / 5001 — ASP.NET Core (HTTP/HTTPS)
  • localhost:8080 — Various tools and containers
  • localhost:8888 — Jupyter Notebook

Security

Services bound to localhost are only accessible from the local machine, not from the network. However, web browsers can access localhost, which means a malicious website could attempt to interact with local development servers (localhost bypasses same-origin policy exceptions in some browsers).

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