Practice And Capstone
Where to Go Next
Module 2 signals, framing, Ethernet, switching
JrCodex·7 min read
Jr Codex Computer Networks Notes
Level: All levels Prerequisites: Chapter 3: Capstone Time to complete: ~15 minutes
Table of Contents
- What This Curriculum Covered
- The Six Ideas Worth Keeping
- Where This Sits in the Jr Codex Path
- Interview Preparation
- Directions for Depth
- A Final Word
1. What This Curriculum Covered
The Arc
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Module 1 layering, encapsulation, measurement
Module 2 signals, framing, Ethernet, switching
Module 3 IP addressing, routing, NAT and DNS's
supporting cast
Module 4 ports, UDP, TCP, congestion control
Module 5 DNS, HTTP, real-time protocols, email
Module 6 threats, cryptography, TLS, firewalls
Module 7 load balancers, CDNs, cloud, meshes
Module 8 diagnosis, and building it yourself
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The Shape of It
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Modules 1-5 follow a packet UP the stack, each
layer solving a limitation of the one below.
Modules 6-7 are what you build ON that stack once
the network is hostile and one machine is not
enough.
Module 8 is the skill the rest exists to enable:
looking at a broken system and knowing where to
look first.
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2. The Six Ideas Worth Keeping
1. EACH LAYER SOLVES THE ONE BELOW IT
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Links lose bits, so framing and checksums. Links
reach one hop, so addressing and routing. Packets
arrive damaged and out of order, so reliability.
Meet a new protocol and ask: what limitation does
this address? That is usually its whole reason
for existing.
2. LATENCY IS NOT BANDWIDTH
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Round trips dominate perceived speed. Propagation
delay is physics and can only be reduced by
reducing distance.
Nearly every performance technique in Modules 5
and 7 is an attack on round trips.
3. THE NETWORK IS HOSTILE
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Every core protocol was designed among trusted
parties. Assume interception, and secure the
CONNECTION rather than the network.
And confidentiality without authentication is
worthless.
4. FAILURE IS NORMAL, NOT EXCEPTIONAL
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Packets are lost. Links go down. Services are
slow. TCP, retries with jitter, circuit breakers
and health checks all exist because failure is
the steady state, not an event.
5. STATE IS THE EXPENSIVE THING
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Stateless is easier to scale, load balance,
restart and reason about. NAT, sticky sessions
and long-lived connections are all awkward
precisely because they hold state somewhere it
complicates everything else.
6. DIAGNOSE BOTTOM-UP
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The first layer that fails localises the problem
and makes everything above it irrelevant.
It is the single most transferable habit in this
curriculum, and it applies far outside networking.
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3. Where This Sits in the Jr Codex Path
The Systems Strand
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Python the tool
↓
DSA algorithms and structures
↓
┌────────────────┬───────────────────────┐
│ Computer │ DBMS │
│ Networks │ │
│ (this one) │ │
└────────────────┴───────────────────────┘
the two systems subjects every backend
engineer is expected to know
These sit alongside the AI strand — Data Science,
ML, AI, Deep Learning, NLP/LLM, Generative AI,
Agentic AI.
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What Connects to What
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DSA Module 9 (Graphs) ──► routing algorithms,
Module 3, Chapter 4
DBMS Module 9 ──► why distributed
transactions cost
what they do; this
curriculum explains
the round trips
DBMS Module 8 ──► write-ahead logging;
the same "log the
intent" idea as
reliable protocols
Gen AI / Agentic AI ──► the production
chapters assume HTTP,
latency, caching and
retries from here
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4. Interview Preparation
What Is Actually Asked
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ALMOST CERTAINLY
What happens when you type a URL and press
enter? ── the whole curriculum in one question
TCP vs UDP, and when to use each (Module 4)
The OSI layers (Module 1)
How does DNS resolution work (Module 5)
What is a three-way handshake (Module 4)
COMMON AT MID-LEVEL
Subnetting arithmetic (Module 3, Chapter 1)
HTTP status codes and idempotency
(Module 5, Chapter 2)
How does HTTPS work (Module 6, Chapter 3)
NAT, and what it breaks (Module 3, Chapter 5)
SENIOR
Design a system to serve X users globally
Debug this: describe the symptom, get the
method (Module 8, Chapter 2)
Congestion control, and why loss means slow
down (Module 4, Chapter 4)
Load balancer choices and health check design
(Module 7, Chapter 1)
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THE URL QUESTION, ANSWERED WELL
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It is a chance to walk the whole stack. A strong
answer names the layer at each step:
1. Browser cache, then OS resolver, then DNS
recursion — root, TLD, authoritative
(Module 5, Ch.1)
2. ARP for the gateway, because the destination
is off-subnet (Module 2, Ch.4)
3. TCP three-way handshake — one round trip
(Module 4, Ch.3)
4. TLS handshake — one more, with certificate
validation (Module 6, Ch.3)
5. HTTP request with the Host header
(Module 5, Ch.2)
6. Routing hop by hop: TTL decrements, MACs
change, IPs do not (Module 3, Ch.3)
7. Possibly a CDN edge, a load balancer, several
services (Module 7)
8. Response, caching headers, rendering
The depth of that answer, and the ability to go
deeper on any step, is what distinguishes
candidates.
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5. Directions for Depth
IF YOU BUILD APPLICATIONS
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→ Modules 4, 5 and 8. Understand your HTTP
client's connection pooling, timeouts and retry
behaviour — most application "network problems"
live there.
IF YOU WANT SRE OR INFRASTRUCTURE
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→ Modules 6, 7 and 8 in depth. Then run things:
build a VPC by hand, configure a load balancer,
break it deliberately and diagnose it. Learn
BGP properly.
IF YOU WANT SECURITY
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→ Module 6, then applied practice. Set up a lab,
capture your own traffic, run the attacks
against your own systems. The OWASP guidance
for the application layer, and the TLS RFCs for
the protocol layer.
IF YOU WANT PROTOCOL DEPTH
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→ Read the RFCs. They are more readable than
their reputation: RFC 9293 (TCP), RFC 9110-9114
(HTTP), RFC 8446 (TLS 1.3), RFC 9000 (QUIC).
→ Then implement one. Module 8's capstone,
extended to HTTP/2 framing, teaches more than
any amount of reading.
THE HABIT WORTH BUILDING
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Capture your own traffic, regularly.
Open Wireshark while doing something ordinary —
loading a page, sending an email, joining a call
— and read what actually happens.
Everything in this curriculum is observable on
your own machine, right now. Very few subjects
offer that.
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6. A Final Word
Networking has a reputation for being a large pile of acronyms to memorise. It is not. It is one problem — get this data to that machine reliably, quickly and safely, over infrastructure nobody controls — attacked in layers, where each layer exists because of a specific limitation of the one beneath it.
Held that way, the acronyms stop being a list. TCP is what you get when IP loses packets. TLS is what you get when the network is hostile. A CDN is what you get when the speed of light is the bottleneck. QUIC is what you get when TCP's ordering guarantee becomes the problem rather than the solution. Every one of them is an answer to a question the previous layer asked.
The specific technologies will change. HTTP/3 will be superseded, IPv6 will finally finish arriving, something will replace the CA model. What will not change is the reasoning: latency is not bandwidth, failure is normal, the network is hostile, and the first layer that fails is where to look.
That reasoning is what makes someone useful when a system is broken at three in the morning, and it is what this curriculum was actually for.
→ Pair this with the DBMS Notes — the other systems subject, and the one whose distributed chapters depend on everything here.
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