Port · ArcLibrary

核心 · Key Idea

In one line: A port is a 16-bit integer (0–65535). IP finds the host; port finds the service on that host. A connection is uniquely identified by a 5-tuple: protocol + source IP + source port + destination IP + destination port.

What it is#

A host might run web, SSH, database, Redis simultaneously… how does the OS know who an incoming packet belongs to? The port.

Visit https://example.com
  → default destination port 443 (HTTPS)
  → your machine picks a free local port as the source (e.g. 53210)
  → server sees "dst 443" and routes to nginx

Analogy#

打个比方 · Analogy

IP = office building address; port = extension at a specific tenant's front desk. One building hosts many companies; each has its own extension.

Key concepts#

Well-knownWell-known

0–1023, reserved for standards (HTTP 80, HTTPS 443, SSH 22, DNS 53). On Linux, non-root can't bind by default.

RegisteredRegistered

1024–49151, vendor-registered with IANA (MySQL 3306, PostgreSQL 5432, Redis 6379).

EphemeralEphemeral

49152–65535 (Linux default 32768–60999); allocated by OS for outbound connections.

5-tuple5-tuple

(protocol, src IP, src port, dst IP, dst port) — uniquely identifies a connection.

Listen vs connectListen vs Connect

Server listens on a port; the client's local port is auto-assigned by the OS at connect time.

How it works#

The same server port can be connected by many clients at once — different 5-tuples (different src IP / port) = different connections.

Practical notes#

netstat -tlnp / ss -tlnp: which ports am I listening on?
lsof -i :3000: who's using port 3000?
Bind 0.0.0.0 vs 127.0.0.1: the former exposes externally, the latter is local-only — mind production.
Port conflicts: before launching a service, ss -ltn | grep :3000; if taken, change port or kill the process.
Below 1024 needs root by default. setcap or iptables redirect 80→8080 are common workarounds.

Easy confusions#

One port, many connections

Server listens on 80, **N clients connect**.
Each connection's 5-tuple differs = different sockets.

Port conflict

**Two processes** both `listen` on 80.
The second errors `Address already in use`.