networks/ip-addresses-and-ports-the-street-address-and-the-apartment-number.md

IP addresses and ports: the street address and the apartment number

Networks · 10 min read ·

Every message on a network needs two things to arrive: an IP address to find the right computer, and a port to find the right program on it. Think street address and apartment number.

This guide explains how IPv4 addresses are built, why 192.168.x.x shows up in every home, what 127.0.0.1 (localhost) really means, and which programs usually live on ports 22, 80, 443, 5432 and 3000. Check any address yourself, then send packets to a server and see which app answers.