Computer Networks

Foundations Of Networking

The OSI and TCP/IP Models

OSI a 7-layer reference model, designed by

JrCodex·6 min read

Jr Codex Computer Networks Notes

Level: Beginner Prerequisites: Chapter 1: What a Network Is and Why Layers Time to complete: ~20 minutes

Table of Contents

  1. Two Models, One Reality
  2. The Seven OSI Layers
  3. The TCP/IP Model
  4. Mapping Them Together
  5. Which One to Use
  6. Talking About Layers in Practice
  7. Summary & Next Steps

1. Two Models, One Reality

The Situation
─────────────────────────────────────────
  OSI    a 7-layer reference model, designed by
         committee in the 1980s as the intended
         standard.
         Excellent for TEACHING and for precise
         vocabulary.
         Almost nothing implements it directly.

  TCP/IP a 4-layer model that describes what the
         internet ACTUALLY does.
         Built from working code, then documented.
         This is what runs.

  You need both: OSI for the vocabulary everyone
  uses, TCP/IP for what is actually happening.
─────────────────────────────────────────

2. The Seven OSI Layers

Top to Bottom
─────────────────────────────────────────
  7  APPLICATION   what the user's software does
                   HTTP, DNS, SMTP, SSH
                   Module 5

  6  PRESENTATION  data format: encoding,
                   compression, encryption
                   TLS sits roughly here
                   Module 6

  5  SESSION       establishing, managing and
                   ending conversations
                   The layer with the least
                   independent existence today

  4  TRANSPORT     process-to-process delivery,
                   reliability, ordering
                   TCP, UDP — Module 4

  3  NETWORK       host-to-host across networks;
                   addressing and routing
                   IP, ICMP — Module 3

  2  DATA LINK     node-to-node on one link;
                   framing, MAC addressing
                   Ethernet, wifi — Module 2

  1  PHYSICAL      bits as signals on a medium
                   cables, radio, voltages
                   Module 2
─────────────────────────────────────────

A Mnemonic Worth Having
─────────────────────────────────────────
  Bottom-up:
    Please Do Not Throw Sausage Pizza Away
    Physical Data-link Network Transport Session
    Presentation Application

  Trivial, and it survives interviews.
─────────────────────────────────────────

Where the Numbers Get Used
─────────────────────────────────────────
  Layer numbers are everyday vocabulary in
  networking and operations:

    "a layer 2 switch"      forwards by MAC address
    "a layer 3 device"      routes by IP
    "a layer 4 load
     balancer"              balances by IP and port
    "a layer 7 load
     balancer"              balances by URL, header
                            or cookie
    "a layer 7 attack"      targets the application,
                            not the link

  This is the main practical reason to know OSI.
─────────────────────────────────────────

3. The TCP/IP Model

Four Layers
─────────────────────────────────────────
  APPLICATION   OSI layers 5, 6 and 7 combined
                HTTP, DNS, SMTP, TLS

  TRANSPORT     = OSI layer 4
                TCP, UDP, QUIC

  INTERNET      = OSI layer 3
                IP, ICMP, ARP
                (sometimes called the network
                 layer)

  LINK          OSI layers 1 and 2 combined
                Ethernet, wifi, PPP
                (also called network access)
─────────────────────────────────────────

Why It Collapsed the Layers
─────────────────────────────────────────
  OSI's session and presentation layers never
  developed independent implementations. In
  practice, applications handle their own sessions,
  and encoding and encryption live in libraries the
  application calls.

  Similarly, physical and data link are inseparable
  in real hardware: an Ethernet card implements
  both, and you cannot buy one without the other.

  TCP/IP merged them because the split described
  nothing real.
─────────────────────────────────────────

THE HOURGLASS
─────────────────────────────────────────
  Many applications ────► HTTP SMTP DNS SSH ...
        │
  Two transports  ──────► TCP    UDP
        │
  ONE network layer ────►      IP        ◄── the
        │                                    waist
  Many links ───────────► Ethernet wifi 5G fibre

  This is the most important structural fact about
  the internet.

  ANY application over ANY link, because everything
  agrees on IP in the middle. Add a new physical
  medium and every existing application works over
  it, unchanged, immediately.

  It is why the internet absorbed wifi, then
  mobile, then fibre, with no application rewrites.
─────────────────────────────────────────

4. Mapping Them Together

Side by Side
─────────────────────────────────────────
   OSI                 TCP/IP        Examples
  ─────────────────────────────────────────
  7 Application  ┐
  6 Presentation ├──► Application   HTTP, DNS,
  5 Session      ┘                  TLS, SMTP
  ─────────────────────────────────────────
  4 Transport    ───► Transport     TCP, UDP,
                                    QUIC
  ─────────────────────────────────────────
  3 Network      ───► Internet      IP, ICMP,
                                    ARP
  ─────────────────────────────────────────
  2 Data Link    ┐──► Link          Ethernet,
  1 Physical     ┘                  wifi, fibre
  ─────────────────────────────────────────

Layer Devices and Addresses
─────────────────────────────────────────
  LAYER  ADDRESS        UNIT      DEVICE
  ─────────────────────────────────────────
    7    URL            data      proxy, gateway
    4    port number    segment   L4 load balancer
    3    IP address     packet    ROUTER
    2    MAC address    frame     SWITCH
    1    none           bit       hub, repeater,
                                  cable

  Learn this table. It answers a large share of
  networking questions on its own, and the
  address/unit/device columns recur throughout the
  curriculum.
─────────────────────────────────────────

5. Which One to Use

The Honest Guidance
─────────────────────────────────────────
  USE TCP/IP to understand what actually happens.
  Four layers, real protocols, real code.

  USE OSI NUMBERS to talk to other people. "Layer
  7 routing" and "layer 2 loop" are the industry's
  shared vocabulary and will not be replaced.

  KNOW OSI'S SEVEN for exams and interviews, which
  ask about it far more than production ever does.
─────────────────────────────────────────

The Trap to Avoid
─────────────────────────────────────────
  Do not try to place every protocol in exactly one
  OSI layer. Many genuinely do not fit:

    TLS      between transport and application;
             called layer 6 by convention, but it
             is a transport-layer protocol by
             function
    ARP      maps IP to MAC — layer 2? 3? Both.
    QUIC     transport features implemented over
             UDP inside the application process
    MPLS     explicitly called "layer 2.5"

  The model is a teaching device. Reality is
  messier, and protocols are designed to solve
  problems rather than to fit diagrams.
─────────────────────────────────────────

6. Talking About Layers in Practice

Diagnosing by Layer — the Real Payoff
─────────────────────────────────────────
  When something is broken, work UP the stack. The
  first failure tells you where to look.

  L1  Is the cable in? Is the interface up?
        ip link show
  L2  Can I reach the local gateway's MAC?
        arp -a
  L3  Can I reach an IP address?
        ping 8.8.8.8
  L4  Can I open a connection to that port?
        nc -zv host 443
  L7  Does the application respond correctly?
        curl -v https://host

  Each step assumes the ones below it worked, which
  is exactly Chapter 1's layering. Module 8 turns
  this into a full diagnostic method.
─────────────────────────────────────────

Why This Ordering Saves Time
─────────────────────────────────────────
  "The website is down" has causes at every layer.

  Testing top-down means guessing. Testing
  bottom-up means the FIRST failure localises the
  problem, and everything above it is irrelevant
  until that is fixed.

  ping works but curl fails ──► the network is
  fine; the problem is layer 4 or above.
  ping fails ──► stop looking at the application
  entirely.
─────────────────────────────────────────

7. Summary & Next Steps

Key Takeaways

  • OSI is a seven-layer teaching model that supplies the industry's vocabulary; TCP/IP is a four-layer description of what actually runs.
  • TCP/IP merged OSI's session and presentation layers because they never had independent implementations, and merged physical and data link because real hardware does too.
  • The hourglass — many applications, two transports, one IP, many links — is why any application works over any new medium without modification.
  • Diagnose bottom-up: the first layer that fails localises the problem, and everything above it is irrelevant until that layer works.

Concept Check

  1. Why did TCP/IP collapse OSI's seven layers into four?
  2. Explain the hourglass shape and what property of the internet it produces.
  3. Why is ping succeeding while curl fails a useful diagnostic result rather than a confusing one?

Next Chapter

→ Chapter 3: Encapsulation

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