Lesson 1: IP Addressing and Subnetting
This lesson answers the most basic and most repeated interview question: how do we identify a host on a network? You will see why IPv4 has the structure it does, why the address is split into network and host bits, and why subnetting was introduced once classful addressing started wasting space. The lesson walks through binary representation, subnet masks, CIDR notation, and a clean method for slicing an address space without a calculator. You will practice common interview problems such as how many usable hosts a /27 carries, how to plan addressing for four offices of unequal size, and how to identify the broadcast address of any given subnet. The why first framing makes subnetting feel like dividing a piece of land into plots, not memorizing power of two tables. By the end of this lesson, you will be able to subnet on a whiteboard while talking through your reasoning, which is exactly what interviewers want to watch.
Lesson 2: How Two Computers Talk When They Are in the Same Network
The simplest case in networking is also the one most candidates explain badly. This lesson walks you through what happens between two laptops sitting on the same switch. You will learn why the sending host needs the destination MAC address, why ARP exists, how the ARP request is broadcast, and why the reply is unicast. You will see how the frame is built layer by layer and what leaves the network interface card. The lesson uses an EVE-NG capture so you can watch the actual frames in flight. By the end you will be able to draw the same network conversation step by step and answer follow up questions such as why ARP entries time out, what happens if two hosts have the same IP, and what the difference is between an ARP request and a gratuitous ARP.
Lesson 3: How Two Computers Talk When They Are in Different Networks
The moment the destination IP is on a different network, everything changes. This lesson shows why the source host stops looking for the destination MAC and instead looks for the default gateway. You will learn how the IP header stays the same while the layer 2 header is rewritten at every router, why the TTL decreases hop by hop, and how the reverse path works. The interviewer will often follow up by asking what happens if there is no default gateway, what happens if the gateway is down, and how a host knows the destination is *different*. All of those answers are covered here. The whiteboard sketch you will learn to draw in this lesson is one of the most reused diagrams in any networking interview.
Lesson 4: How to Avoid Loops in Layer 2 Networks (Spanning Tree Protocol)
Switches forward broadcasts on every port. The moment you add a redundant cable between two switches, a broadcast can circle forever and bring the network down. This lesson explains why this problem exists, why we still want redundancy anyway, and how Spanning Tree Protocol solves both at once. You will learn the role of the root bridge, how bridge priority and MAC address decide the election, what the port roles are, and how BPDU exchange keeps the topology stable. You will see a topology in EVE-NG where one port goes into blocking state, then watch what happens when a link fails and the blocked port wakes up. Interview questions on convergence time, the difference between STP, RSTP, and MSTP, and the meaning of *bridge priority* are all covered.
Lesson 5: Why OSI Model if We Are Already Running TCP/IP Everywhere
This is one of the most asked and least understood interview questions. The lesson explains the honest answer that the OSI model was a teaching and reference framework, while the TCP/IP stack is what the internet actually runs. You will learn what each OSI layer is supposed to do, where the TCP/IP stack collapses or expands those layers, and how to use the OSI model as a troubleshooting tool even though no real device implements it cleanly. The lesson also covers the common interview trap of asking *which layer does X happen at*, with a clear reasoning approach you can apply to any new protocol the interviewer throws at you.
Lesson 6: Why Do We Need MAC Address if We Already Have IP Address
This is a question most candidates have never seriously thought about. The lesson explains that IP is a logical address used to find a network, while MAC is a physical address used to deliver inside that network. You will see why a host on the same LAN never uses the destination IP for frame delivery, why MAC addresses are flat while IP addresses are hierarchical, and what would break if one were removed. The whiteboard explanation prepared in this lesson is the answer that interviewers remember.
Lesson 7: How Traceroute Works
Most candidates can use traceroute. Very few can explain why it works. This lesson shows how the tool sends probes with increasing TTL values, why each router along the path replies with an ICMP time exceeded message, and how the trace builds itself hop by hop. You will see what happens when a router blocks ICMP, why traceroute output sometimes shows asterisks, and how to read the output to spot routing problems. The lesson also covers the difference between traceroute and tracert, and why Windows and Linux use different default protocols.
Lesson 8: Significance of TCP Sequence and Acknowledgement Numbers
TCP is reliable because it counts every byte. This lesson opens with the why: a layer 4 protocol on an unreliable network has to detect loss and recover from it. You will see how the initial sequence number is chosen, how each segment advances the sequence count, how the receiver acknowledges, and what happens during retransmission. The lesson also covers selective acknowledgement, sliding window behavior, and the famous three way handshake. Interviewers commonly follow up with questions on flow control, congestion control, and the difference between TCP and UDP. All of those threads are covered here. For deeper transport layer coverage, pair this lesson with
TCP From Scratch.
Lesson 9: Why Behind MTU and MSS
Why do two seemingly similar values exist? Because they live at different layers. This lesson explains that MTU is a layer 3 limit measured by the network interface, while MSS is a layer 4 limit advertised by TCP during the handshake. You will see how fragmentation works when a packet is larger than the path MTU, why fragmentation is bad for performance, and how path MTU discovery solves the problem. The lesson also covers the practical implications when running tunneling protocols such as GRE or IPSec, where the additional header can break large packets in silent and confusing ways.
Lesson 10: Why is DNS So Important for the Internet?
Without DNS the internet would still work, but no one could use it. This lesson explains why human readable names are needed, how the DNS hierarchy is structured, what root, top level, and authoritative servers do, and how recursive resolution flows from a stub resolver all the way back to the answer. You will see why caching makes the system fast, why TTL values matter, and how reverse DNS lookups are used. Common interview follow ups include the difference between A and AAAA records, what a CNAME does, why MX records exist, and what happens when a DNS server is unreachable.
Lesson 11: What Happens When You Type Google.com?
The most asked end to end interview question in the industry. This lesson walks every single step. You will see the browser check its cache, the OS check its cache, the resolver query its upstream, the recursive query reach the authoritative server, the IP get returned, the TCP three way handshake begin, the TLS negotiation finish, the HTTP request get sent, the response come back, and the page render. The whiteboard explanation you will rehearse here is the answer that sets the tone for the rest of the interview. If you cannot answer this cleanly, interviewers worry. If you can, they relax and ask the deeper questions.
Lesson 12: How to Connect Your Network to the Internet
Real networks need a path to the public internet. This lesson covers why private IP addressing exists, why NAT was invented, how PAT lets many private hosts share one public address, and what an ISP edge typically looks like. You will see how default routes are advertised, why a redundant uplink matters, and what the difference is between a static default and a dynamic default learned from the ISP. Interview questions on NAT types, NAT versus PAT, and what happens to a TCP session during NAT translation are all covered.
Lesson 13: OSPF Link State Nature — How Does it Avoid Loops?
Why is OSPF called a link state protocol, and why does that make it loop free? Because every router builds the same complete map of the network and then runs the SPF algorithm locally. This lesson explains how link state advertisements flood, how the link state database is built, and how Dijkstra produces a tree with the router itself as the root. You will see why distance vector protocols can suffer from loops and counting to infinity while link state protocols cannot. The lesson also covers OSPF area design at an interview level, so you can answer why areas exist and what the backbone area is for. For full OSPF depth, follow with
OSPF from Scratch to Design.
Lesson 14: Can You Design a Network for a Small Office?
The interview design question. This lesson walks you through how to size the network, how to choose access and distribution layers for a small site, where to place the router, how to plan IP addressing, how to handle WiFi, how to connect to the internet, and how to leave room for growth. You will see how a clean two layer or three tier design is drawn on the whiteboard, and what trade offs are acceptable for a small office that will not need a full enterprise architecture. This lesson alone has decided many interviews, because design questions reveal how a candidate thinks.
Lesson 15: How to Troubleshoot a Network
Why a method matters more than a tool. This lesson teaches the layer by layer troubleshooting approach starting from layer 1 cable checks, moving up through ARP and IP reachability with ping and traceroute, then into transport and application layer checks. You will learn how to talk through your reasoning out loud so the interviewer can follow your thinking, which is the part that scores marks. The lesson covers classic interview scenarios such as *the user says the internet is slow*, *one site cannot reach another*, and *DNS sometimes fails*, and walks through how to structure your answer for each.
