Engineering ReferenceEngineering Reference

Networking

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Networking moves bytes between machines reliably and securely. A working engineer needs to read packets, understand the stack layer by layer, and reason about latency, throughput, and loss.

Overview

Two reference models are everywhere: the 7-layer OSI model and the simpler 4-layer TCP/IP model. Both describe the same job — framing, addressing, delivery, ordering, and presenting bytes — split across cooperating layers.

OSI & TCP/IP

  • 1 Physical — copper, fiber, RF.
  • 2 Data link — Ethernet, Wi-Fi, MAC, VLANs.
  • 3 Network — IPv4 / IPv6, routing.
  • 4 Transport — TCP, UDP, QUIC.
  • 5–7 Session / Presentation / Application — HTTP, DNS, TLS, SMTP, MQTT.

Addressing & Subnetting

  • IPv4: 32 bits; private ranges 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16.
  • CIDR notation — /24 = 256 addresses, /30 = 4 (2 usable).
  • IPv6: 128 bits; link-local fe80::/10, ULA fc00::/7.
  • MAC: 48 bits; first 24 = OUI.
  • NAT, PAT; loopback 127.0.0.1.

Common Protocols

  • DHCP (67/68) — address assignment.
  • DNS (53) — name resolution, A/AAAA/CNAME/MX/TXT.
  • HTTP/HTTPS (80/443) — web.
  • SSH (22), RDP (3389).
  • SMTP/IMAP/POP3 (25/143/110) — mail.
  • NTP/PTP — time sync.
  • SNMP (161/162) — device management.
  • Industrial: Modbus TCP (502), EtherNet/IP (44818/2222), OPC UA (4840), PROFINET.

Routing & Switching

  • Switches forward frames by MAC; build CAM/MAC tables.
  • VLANs (802.1Q) segment broadcast domains.
  • STP/RSTP (802.1D/w) prevents loops; PortFast, BPDU Guard.
  • Routers forward packets by longest-prefix IP match.
  • Routing protocols: OSPF, EIGRP, BGP, IS-IS.

Security

  • Firewall rules (stateful), zones / DMZ.
  • TLS 1.2/1.3, certificate chains, PKI.
  • VPN — IPsec, WireGuard, OpenVPN.
  • 802.1X port authentication.
  • Zero Trust — verify every request, segment everything.
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