Linux Networking I: TCP/IP, CIDR Subnetting, ip a & ip r | Zero to LLMOps (Day 15)

Day 15 of a free 180-day DevOps course. We have arrived at Week 3! For the first two weeks, everything we built lived entirely inside one standalone Linux kernel: processes, storage with LVM, systemd services, and logrotate retention. But a server in isolation is just a heater in a data center. What we cover today: • The Production TCP/IP Model: Why engineers use the pragmatic 4-layer stack (Link, Internet, Transport, Application) over the academic 7-layer OSI model. • Packet Encapsulation: How application payloads get wrapped in TCP, IP, and Ethernet headers on transmission, and unwrapped in reverse upon arrival. • Observing the Handshake: Using curl -v to watch DNS resolution, TCP SYN handshakes, TLS negotiation, and HTTP transactions execute in strict sequence. • IPv4 Addressing Anatomy: 32 bits, four 8-bit octets, and the division between network prefix and host ID. • CIDR Subnetting Math: How /24, /25, /26 masks slice host capacities in half, and calculating network addresses, broadcast addresses, and usable host counts. • RFC 1918 Private Ranges: The unroutable IP blocks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) powering VPCs and corporate clouds. • The 169.254.x.x Signal: Why an APIPA link-local address on a server is an immediate alert of DHCP failure. • Retiring Deprecated net-tools: Replacing ifconfig, route -n, and arp with modern iproute2 commands (ip a, ip link, ip r, ip neigh). • ARP Mechanics: How software IP addresses resolve to physical 48-bit MAC addresses on local Ethernet broadcast domains. • Kernel Routing Table: Default gateways (0.0.0.0/0) and the non-negotiable rule of Longest Prefix Match. • Route Simulation: Using ip route get (destination-ip) to test kernel forwarding decisions without sending a single packet on the wire. • Production Incident: How a subtle gateway typo in a static IP profile caused a silent network isolation incident. ⏱ Chapters 00:00 Intro: Week 3 Begins & Why a Server in Isolation Is Just a Heater 01:21 Recall Warm-Up: Days 11-14 (Wants vs Requires, journald Persistence, copytruncate, LVM & Log Triage) 02:57 Part A: The Practical 4-Layer TCP/IP Model (Link, Internet, Transport, Application) 04:17 Packet Encapsulation & Decapsulation (Headers on the Way Down, Unwrapping on the Way Up) 04:58 Demo: curl -v Observing Strict Layer Handshake in Real-Time 05:23 Part B: IPv4 Addressing & CIDR Subnetting Math (/24, /25, /26) 06:22 Reserved IPs: Network Address, Broadcast & Usable Host Calculation 07:03 RFC 1918 Private Ranges & 169.254.x.x Link-Local Fallback (APIPA) 08:05 Demo: ipcalc Validating Subnet Bounds & Netmasks 08:26 Part C: iproute2 vs Deprecated net-tools (ip a, ip link, ip r, ss) 09:22 ARP (Address Resolution Protocol): How Software IP Maps to Hardware MAC 10:09 Demo: ip -br link & ip -br address (Clean One-Line State Tables) 10:27 Part D: Linux Kernel Routing Table & The Longest Prefix Match Rule 11:41 Simulating Packet Routing with ip route get 11:56 Production Incident: The Subnet Gateway Typo 12:14 LLMOps Connection: RoCE, InfiniBand & Multi-Node NCCL Communication Hangs 12:44 Demo: ip route get 8.8.8.8 vs Loopback 127.0.0.1 13:21 Lab Walkthrough: Day 15 Lab Overview 13:28 Lab Part 1: Interface & Address Discovery (ip link show, ip a) 13:51 Lab Part 2: Subnetting Calculations by Hand & ipcalc Cross-Check 14:46 Lab Part 3: ARP Cache & Routing Table Inspection (ip neigh, ip route get) 15:02 Harmless Static Route Drill: Add, Verify & Delete 15:22 Lab Part 4: Layer-by-Layer Packet Path Narration (ping, traceroute / tracepath) 16:12 Recap: TCP/IP Stack, Encapsulation, CIDR & iproute2 Commands 17:05 Homework & 3 Closing Recall Questions 17:45 Day 16 Preview: Networking II (NetworkManager, nmcli, DNS & ss) Definition of Done: 1. Identified all local network interfaces, MAC addresses, and active IPv4/CIDR assignments using ip link and ip a. 2. Completed subnet calculations on paper and verified network, broadcast, and host ranges using ipcalc. 3. Inspected ARP neighbors with ip neigh and simulated kernel routing decisions with ip route get. 4. Added, verified, and safely removed a temporary static route without impacting production traffic. 5. Traced packet delivery hops and narrated layer-by-layer encapsulation from application to wire. 📚 FREE COURSE MATERIAL — every lesson, lab and answer key: https://github.com/mahadikabhijeet/ze... All 185 written lessons, labs, and study guides are on GitHub, free and open. ◀ Previous — Day 14: Phase 1 Review Day (Cumulative Quiz & Catch-Up on Days 8-13) ▶ Next — Day 16: Networking II (DNS, Sockets, ss, curl, dig) 🔔 Subscribe for the full path from zero to LLMOps. #DevOps #Linux #Networking #TCPIP #Subnetting #CIDR #iproute2 #LearnLinux #SysAdmin #ZeroToLLMOps