Localhost and Development Server URLs

Localhost URLs

localhost is a hostname that resolves to the loopback network interface, allowing a computer to communicate with itself. It is fundamental to local development.

Localhost Resolution

localhost     → 127.0.0.1 (IPv4) or ::1 (IPv6)
127.0.0.1     → Direct IPv4 loopback
[::1]         → Direct IPv6 loopback
0.0.0.0       → All network interfaces (bind address, not a valid URL host)

Common Development URLs

http://localhost:3000          → React / Next.js / Express
http://localhost:4200          → Angular CLI
http://localhost:5173          → Vite
http://localhost:8080          → Spring Boot / Tomcat / Go
http://localhost:8000          → Django / PHP artisan
http://127.0.0.1:5432         → PostgreSQL
http://127.0.0.1:6379         → Redis
http://localhost:27017         → MongoDB

localhost vs 127.0.0.1

Aspect	localhost	127.0.0.1
Type	Hostname	IP address
Resolution	DNS lookup (usually /etc/hosts)	Direct, no DNS
IPv6	May resolve to ::1	IPv4 only
Cookies	Domain: localhost	Domain: 127.0.0.1
Speed	Slightly slower (DNS)	Slightly faster
CORS	Treated as different origins if mixed	—

IPv6 Loopback

IPv6 loopback ::1 must be enclosed in brackets in URLs:

http://[::1]:3000/api/health

Binding: 0.0.0.0 vs 127.0.0.1

When starting a development server, the bind address matters:

• 127.0.0.1 — Only accessible from the same machine
• 0.0.0.0 — Accessible from any network interface (other machines on the LAN can connect)

# Only local access
next dev --hostname 127.0.0.1

# Accessible from network (e.g., testing on phone)
next dev --hostname 0.0.0.0

HTTPS on Localhost

For local HTTPS development, tools like mkcert create locally-trusted certificates:

mkcert localhost 127.0.0.1 ::1
# Creates localhost.pem and localhost-key.pem