Internet Cheatsheet

── The layers ───────────────────────────
Application  HTTP, DNS, SSH    (meaning)
Transport    TCP/UDP           (delivery, ports)
Internet     IP                (addressing, routing)
Link         Ethernet, WiFi    (next physical hop)
data = envelopes in envelopes; routers read IP only

── Addresses ────────────────────────────
IPv4: 93.184.216.34 (4 bytes, scarce)
IPv6: 2606:2800:... (16 bytes, plentiful)
private: 10.x | 172.16-31.x | 192.168.x
127.0.0.1 = localhost | NAT: home shares one public IP

── DNS ──────────────────────────────────
browser cache -> OS -> resolver -> root -> TLD -> authoritative
A=IPv4  AAAA=IPv6  CNAME=alias  MX=mail  TXT=misc  NS=who answers
TTL controls caching -> why changes "propagate"

── TCP vs UDP ───────────────────────────
TCP: handshake (SYN/SYN-ACK/ACK), numbered bytes,
  ACKs, retransmit, ordering -> reliable stream
UDP: fling packets, no promises -> DNS, games, calls
QUIC (HTTP/3): reliability over UDP, no head-of-line block
connection = (src IP, src port, dst IP, dst port)

── Ports worth knowing ──────────────────
22 SSH | 53 DNS | 80 HTTP | 443 HTTPS
5432 Postgres | 3306 MySQL | 6379 Redis | 3000/8080 dev

── HTTP ─────────────────────────────────
GET read | POST create | PUT replace | PATCH edit | DELETE
2xx ok | 3xx redirect | 4xx your fault | 5xx server's fault
200 ok  201 created  301 moved  304 cached  400 bad request
401 unauthenticated  403 forbidden  404 missing
429 rate limited  500 crashed  502/503 backend down
cookies = server-set state the browser echoes back
HTTPS = HTTP in TLS: certs prove identity, math encrypts

# ── Debugging toolbox ───────────────────
ping example.com               # reachable? latency?
traceroute example.com         # the route (tracert on Win)
dig example.com +short         # DNS answer
dig example.com MX             # any record type
dig example.com +trace         # full resolution chain
curl -v https://example.com    # whole HTTP conversation
curl -I https://example.com    # headers only
curl -o /dev/null -s -w '%{time_total}s\n' URL   # timing
curl ifconfig.me               # my public IP
netstat -an | grep LISTEN      # what's listening here
nc -zv host 443                # is that port open?

# ── Reading a failure ───────────────────
# DNS error (NXDOMAIN)      -> name wrong / DNS broken
# connection refused        -> host up, nothing on port
# timeout                   -> firewall or host down
# TLS/cert error            -> expired/wrong-domain cert
# 4xx                       -> fix your request
# 5xx                       -> their server; retry later
# works here, not there     -> caching or DNS, always

# ── Page-load order (perf checklist) ────
# DNS -> TCP -> TLS -> request -> HTML -> subresources
# fewer round trips > smaller bytes
# CDN = closer server; cache-control = skip requests
# defer scripts; browser DevTools Network tab = truth