IP Networking

Core Idea

IP networking moves packets between hosts; protocols such as TCP, SSH, HTTP, DNS, and SMTP build useful conversations on top of that movement.

Layered Questions

When a network task fails, separate the questions:

Name: does DNS resolve the hostname?
Address: which IP address did it resolve to?
Reachability: can packets travel there?
Port: is the service listening on the expected port?
Protocol: did the client and server speak the same protocol?
Application: did authentication, routing, files, or permissions succeed?

Course Commands

host lf2607.kolamayermakers.org
ping -c 3 1.1.1.1
curl -I https://lf2607.kolamayermakers.org/~username/
ssh username@lf2607.kolamayermakers.org

Each command answers a different networking question. Do not use one command as proof for every layer.

Addresses And Ports

IPv4: 203.0.113.10
IPv6: 2001:db8::10
Localhost IPv4: 127.0.0.1
Localhost name: localhost
HTTPS default port: 443
SSH default port: 22

A port identifies a service endpoint on a host. Your course user-managed service listens on a local high port behind the public proxy.

DNS Is Not The Service

DNS can work while HTTP fails. HTTP can fail while SSH works. Ping can fail while HTTPS works. Keep the layers separate.

Common Failures

Name or service not known: hostname did not resolve.
Connection refused: a host answered, but nothing accepted the connection on that port.
Timeout: packets or replies did not arrive before the client gave up.
502: a proxy accepted your request but could not reach the backend service.
Certificate error: TLS identity or trust failed.

Proof Check

For a broken URL, run one DNS command and one HTTP command. Explain which layer each command tested.

Docs Pointers

Run man host, man dig, man ping, man ssh, and man curl.
Read MDN: How DNS works.
Read MDN: What is a URL?.
Read DNS, ICMP, and HTTP for focused protocol notes.
Read IP Addressing Basics for address notation and localhost details.
Read Sockets for how processes use addresses and ports.