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Computer Networks Level 1 Fundamentals

Level 1 fundamentals for computer networks including the OSI model and TCP/IP.

1. OSI Model

1.1 Why Need Layers

Network communication involves many different responsibilities:

  • Creating application data
  • Representing/formatting data
  • Managing communication sessions
  • Providing reliable/end-to-end delivery
  • Identifying destination networks
  • Delivering data on a local network
  • Physically transmitting bits

If one protocol handled everything, networking would become extremely complicated.

Therefore, networking is divided into layers, where each layer has a specific responsibility.

The OSI (Open Systems Interconnection) model is a 7-layer reference model used to understand these responsibilities.

2. The 7 OSI Layers

From top to bottom:

Layer Name Main Responsibility Data Unit
7 Application Network services used by applications Data
6 Presentation Data representation, encryption, compression Data
5 Session Session/communication management Data
4 Transport End-to-end/process-to-process delivery Segment / Datagram
3 Network Logical addressing and routing Packet
2 Data Link Local/node-to-node delivery Frame
1 Physical Transmission of raw bits Bits

Mnemonic:

All People Seem To Need Data Processing

Bottom-up:

Please Do Not Throw Sausage Pizza Away

Understanding the responsibilities is more important than memorizing the mnemonic.

3. Physical Layer

Main responsibility:

The Physical layer handles the actual transmission of raw bits over a physical medium.

Example:

101101001010...

These bits are represented using physical signals.

Depending on the technology, these can be:

  • Electrical signals
  • Radio waves
  • Light signals

Examples:

Ethernet:

Computer
   ↓
Electrical signals
   ↓
Ethernet cable
   ↓
Switch

Wi-Fi:

Computer
   ↓
Radio waves
   ↓
Wi-Fi Access Point

Things associated with Physical Layer:

  • Cables
  • Fiber optic
  • Radio transmission
  • Connectors
  • Signal levels
  • Bit transmission
  • Physical transmission medium
  • Data rate

The Physical layer does not understand:

  • IP addresses
  • MAC addresses
  • TCP
  • HTTP

It deals with the physical transmission of bits.

Data unit:

Bits

The Data Link layer handles node-to-node/local-network communication.

It provides structure to the raw bits received from the Physical layer.

Main responsibilities:

  • Framing
  • MAC addressing
  • Local delivery
  • Error detection
  • Medium access control

Data unit:

Frame

Conceptually:

┌──────────────┬──────────────┬───────────┐
│ Frame Header │     Data     │  Trailer  │
└──────────────┴──────────────┴───────────┘

4.1 MAC Address

A MAC address identifies a network interface at the Data Link layer.

Example:

00:1A:2B:3C:4D:5E

4.2 Switch

A traditional Ethernet switch primarily operates at Layer 2.

It uses MAC addresses to determine where to forward frames.

PC A
  ↓
Switch
  ↓
PC B

Important:

Traditional switch → Layer 2
Layer 3 switch → can also perform Layer 3 routing

5. Network Layer

The Network layer handles communication between different networks.

Example:

Laptop
   ↓
Router
   ↓
Router
   ↓
Server

The Network layer is responsible for:

  • Logical addressing
  • Routing
  • Packet forwarding between networks

The major protocol is:

IP — Internet Protocol

5.1 IP Address

Example:

192.168.1.10

Data unit:

Packet

5.2 Router

A router primarily operates at Layer 3.

It uses the destination IP address and its routing table to determine where the packet should be forwarded.

6. Transport Layer

The Transport layer provides end-to-end/process-to-process communication.

A computer can run many applications simultaneously:

Server
 ├── Web server
 ├── SSH
 ├── Database
 └── Other applications

The Transport layer helps identify the correct application/process using port numbers.

Major protocols:

  • TCP
  • UDP

Responsibilities:

Depending on the protocol:

  • End-to-end delivery
  • Process-to-process communication
  • Port numbers
  • Reliability
  • Ordering
  • Flow control
  • Congestion control
  • Error recovery

Example:

Destination IP + Port
142.250.x.x : 443

The IP address identifies the destination machine/network interface, while the port identifies the destination service/process.

Data units:

TCP:

Segment

UDP:

Datagram

7. Session Layer

The Session layer deals with managing communication sessions/dialogues between applications.

Conceptually:

Establish session
      ↓
Communication
      ↓
Manage/synchronize session
      ↓
Terminate session

Responsibilities traditionally associated with this layer include:

  • Session establishment
  • Session management
  • Session termination
  • Synchronization/checkpoints

Important practical point:

Modern Internet protocols do not always map cleanly to a separate Session layer.

The OSI model is a reference/conceptual model, so don't assume every modern application literally has a separate Session-layer protocol.

For interviews:

Session layer = management of communication sessions/dialogues.

8. Presentation Layer

The Presentation layer deals with how data is represented.

Think:

"How should the data be represented so that communicating systems can understand it?"

Traditionally associated with:

1. Translation / encoding:

Converting between different data representations.

2. Encryption / decryption:

Plaintext
   ↓
Encryption
   ↓
Ciphertext

3. Compression / decompression:

Large data
   ↓
Compression
   ↓
Smaller data

Therefore:

Presentation = representation of data

Encryption and compression are traditionally associated with Layer 6 in the OSI model.

9. Application Layer

The Application layer is closest to applications and provides network services/protocols used by them.

Examples:

  • HTTP
  • DNS
  • SMTP
  • FTP

Example:

GET /index.html

HTTP is an Application-layer protocol.

Important distinction:

The application itself is not necessarily an "Application-layer protocol."

For example:

  • Chrome = application
  • HTTP = Application-layer protocol used by applications

10. Important Layer Identifiers

One of the most useful interview mappings:

MAC address → Layer 2
IP address  → Layer 3
Port number → Layer 4

Think:

MAC → local delivery
IP  → routing/destination network
Port → destination process/service

11. Devices And Layers

Switch:

Traditional Ethernet switch:

Layer 2 → uses MAC addresses

Router:

Router:

Layer 3 → uses IP addresses for routing

Layer 3 Switch:

Can perform both:

  • Layer 2 switching
  • Layer 3 routing

12. OSI Reference Model

Do not say:

"The Internet follows the OSI model exactly."

Instead:

OSI is a reference/conceptual model for understanding network communication.

The Internet primarily uses the TCP/IP protocol suite/architecture.

13. TCP IP Model

13.1 Why TCP IP

The Internet was developed around the TCP/IP protocol suite.

A commonly taught TCP/IP model has 4 layers:

4. Application
3. Transport
2. Internet
1. Network Access / Link

14. TCP IP Application Layer

The TCP/IP Application layer combines three OSI layers:

OSI:
Application
Presentation
Session

        ↓

TCP/IP:
Application

Protocols such as:

  • HTTP
  • DNS
  • SMTP
  • FTP

belong to the TCP/IP Application layer.

Important:

TCP/IP does not require the strict separation of Application, Presentation, and Session that OSI defines.

15. TCP IP Transport Layer

This corresponds directly to the OSI Transport layer.

Major protocols:

  • TCP
  • UDP

Responsibilities include:

  • Process-to-process communication
  • Port numbers
  • Reliability
  • Ordering
  • Flow control
  • Congestion control

16. TCP IP Internet Layer

This corresponds primarily to the OSI Network layer.

Major protocol:

IP

Responsibilities:

  • Logical addressing
  • Routing
  • Packet forwarding
Source IP → Destination IP

A router primarily operates here for routing purposes.

OSI calls this:

Network layer

TCP/IP calls this:

Internet layer

This layer combines the responsibilities represented by:

OSI Data Link
+
OSI Physical

It deals with:

  • Frames
  • MAC addresses
  • Local delivery
  • Physical transmission
  • Network media/access

Examples:

  • Ethernet
  • Wi-Fi

Depending on the textbook, this layer may be called:

  • Network Access layer
  • Link layer

18. OSI TCP IP Mapping

OSI                         TCP/IP
────────────────────────────────────────

Application       ┐
Presentation      ├──────→ Application
Session           ┘

Transport         ───────→ Transport

Network           ───────→ Internet

Data Link         ┐
Physical          ┴──────→ Network Access / Link

Important mapping:

OSI TCP/IP
Application Application
Presentation Application
Session Application
Transport Transport
Network Internet
Data Link Network Access / Link
Physical Network Access / Link

19. Why TCP IP Fewer Layers

TCP/IP doesn't require the same strict separation as OSI.

For example:

OSI:
Application
Presentation
Session

becomes:

TCP/IP:
Application

Similarly:

OSI:
Data Link
Physical

is commonly represented as:

TCP/IP:
Network Access / Link

The point is not that fewer layers automatically makes TCP/IP "better."

The two models simply use different abstractions.

20. TCP IP Scope

A common misconception:

TCP/IP = TCP + IP

This is incorrect.

TCP/IP refers broadly to the Internet protocol suite.

Examples:

Application:

  • HTTP
  • DNS
  • SMTP
  • FTP

Transport:

  • TCP
  • UDP

Internet:

  • IP
  • ICMP

Link:

  • Ethernet
  • Wi-Fi

TCP and IP are important protocols in the suite, but TCP/IP is not literally limited to those two protocols.

21. 4 Layer Vs 5 Layer

You may encounter two versions.

4-layer version:

Application
Transport
Internet
Network Access / Link

5-layer teaching model:

Application
Transport
Network
Data Link
Physical

The 5-layer version separates the TCP/IP Link/Network Access layer into:

Data Link
Physical

This is often done to make networking concepts easier to teach and to compare with OSI.

Don't get confused by the terminology.

Focus on the responsibilities and mapping.

22. Encapsulation

Encapsulation is the process in which data moves down the networking layers and each layer adds its own control information.

Suppose the application produces:

Hello

22.1 Application

Application Data
= "Hello"

22.2 Transport

Suppose TCP is being used.

TCP adds a TCP header:

┌──────────────┬──────────────┐
│ TCP Header   │ Application  │
│              │ Data         │
└──────────────┴──────────────┘

This becomes a:

TCP Segment

22.3 Network

IP adds an IP header:

┌─────────────┬──────────────┬──────────────┐
│ IP Header   │ TCP Header   │ Application  │
│             │              │ Data         │
└─────────────┴──────────────┴──────────────┘

This becomes an:

IP Packet

The Data Link layer adds a frame header and usually a trailer:

┌──────────────┬──────────────────┬─────────────┐
│ Frame Header │    IP Packet     │  Trailer    │
└──────────────┴──────────────────┴─────────────┘

This becomes a:

Frame

22.5 Physical

The frame is transmitted as physical signals representing bits:

Frame
  ↓
Bits
  ↓
Electrical / optical / radio signals

23. Encapsulation Chain

The fundamental chain is:

Application Data
       ↓
    Segment
       ↓
     Packet
       ↓
     Frame
       ↓
      Bits

Or:

Application
     ↓
Transport
     ↓
Network
     ↓
Data Link
     ↓
Physical

24. Nested Structure

The data units are not independent copies.

They are nested inside one another:

Frame
└── Packet
    └── Segment
        └── Application Data

More visually:

┌─────────────────────────────────────────────┐
│ Frame                                       │
│                                             │
│  ┌───────────────────────────────────────┐  │
│  │ IP Packet                             │  │
│  │                                       │  │
│  │  ┌─────────────────────────────────┐  │  │
│  │  │ TCP Segment                     │  │  │
│  │  │                                 │  │  │
│  │  │   Application Data              │  │  │
│  │  │                                 │  │  │
│  │  └─────────────────────────────────┘  │  │
│  └───────────────────────────────────────┘  │
└─────────────────────────────────────────────┘

Therefore:

A packet contains a segment.

A frame contains a packet.

25. Decapsulation

At the receiver, the reverse process occurs.

Bits
 ↓
Frame
 ↓
Packet
 ↓
Segment
 ↓
Application Data

This is called:

Decapsulation

26. Who Removes Which Header

A crucial rule:

Each layer processes the header/trailer belonging to its own protocol.

At the destination:

Data Link:

Processes:

Ethernet/Data Link header + trailer

Leaves:

IP Packet

Network:

Processes:

IP header

Leaves:

TCP Segment

Transport:

Processes:

TCP header

Leaves:

Application Data

So:

Data Link → handles Layer 2 information
Network   → handles IP information
Transport → handles TCP/UDP information
Application → receives application data

The lower layer does not remove a higher layer's header.

For example:

The Data Link layer does not remove the TCP header.

The TCP layer processes the TCP header.

27. Router Processing

A router is particularly important because it does not normally decapsulate all the way up to TCP/Application.

Suppose:

PC → Router 1 → Router 2 → Server

At Router 1, conceptually:

Incoming signals
      ↓
Physical Layer
      ↓
Bits
      ↓
Data Link
      ↓
Frame
      ↓
IP Packet
      ↓
Routing decision
      ↓
New Layer-2 Frame
      ↓
Bits/signals
      ↓
Next hop

A router generally does:

Frame → Packet → Routing → New Frame

It does not normally do:

Frame
 ↓
Packet
 ↓
TCP Segment
 ↓
Application Data

because it does not need to inspect the application data or TCP payload to perform ordinary IP forwarding.

28. Router Lower Layers

Yes.

A router isn't simply a "Layer 3 object."

Its interfaces participate in lower-layer communication too.

Conceptually:

Router
┌─────────────────────┐
│ Layer 3             │ ← IP / Routing
├─────────────────────┤
│ Layer 2             │ ← Frames / MAC
├─────────────────────┤
│ Layer 1             │ ← Signals / Bits
└─────────────────────┘

Therefore, when a frame arrives at a router:

Signals
   ↓
Physical processing
   ↓
Bits / Frame
   ↓
Data Link processing
   ↓
IP Packet
   ↓
Layer 3 routing

The router then sends the packet through another interface:

IP Packet
   ↓
New Layer-2 Frame
   ↓
Physical signals
   ↓
Next hop

Real routers use specialized hardware and buffers, so don't imagine that they literally convert everything into a giant string of 0s in software. This is the conceptual networking model.

29. Frame Change Rationale

Consider:

PC ─── R1 ─── R2 ─── Server

There are multiple local links.

29.1 PC To R1

Source MAC      = PC
Destination MAC = R1

29.2 R1 To R2

Source MAC      = R1
Destination MAC = R2

29.3 R2 To Server

Source MAC      = R2
Destination MAC = Server

Therefore:

The Layer-2 frame is recreated for every hop.

30. IP Vs MAC

This is one of the most important concepts in Level 1.

IP address:

Used for:

Layer-3 routing / determining where the packet ultimately needs to go

MAC address:

Used for:

Layer-2 delivery on the current local link

A useful mental model:

IP → "Which way should the packet go?"

MAC → "How do I deliver it to the next device on this particular link?"

31. Routing And MAC Coordination

Suppose:

PC → R1 → R2 → Server

PC wants to reach the server.

The PC knows the destination IP:

Destination IP = Server IP

It determines that the destination is not directly reachable on its local network, so the next hop is R1.

The Layer-2 frame is then addressed to R1:

Source MAC      = PC
Destination MAC = R1

R1 receives the frame.

It processes the Layer-2 information and extracts the IP packet.

R1 looks at:

Destination IP = Server IP

It consults its routing table:

Destination Network → Next Hop / Interface

Suppose it determines:

Next hop = R2

Now R1 creates a new Layer-2 frame:

Source MAC      = R1's outgoing interface
Destination MAC = R2's interface

The IP packet is carried inside this new frame.

This happens at every router.

32. Separate Decisions Rationale

Because there are two separate decisions.

32.1 Layer 3 Routing

Use the destination IP:

Destination IP
      ↓
Routing table
      ↓
Choose outgoing interface / next hop

32.2 Layer 2 Delivery

Once the outgoing interface/next hop is known:

Next-hop device
      ↓
Find its MAC
      ↓
Build Layer-2 frame
      ↓
Transmit

So:

IP → determines the route / next hop
MAC → performs delivery to that next hop on the selected link

This is why we say:

Routers route using IP addresses.

Even though the actual physical hop-by-hop transmission also involves MAC addresses.

33. Hop By Hop Delivery

For:

PC → R1 → R2 → R3 → Server

Think:

Layer 2:
PC → R1
R1 → R2
R2 → R3
R3 → Server

Each hop has a different frame.

Whereas:

Layer 3:

PC ─────────────────────────→ Server
          Same IP destination

The IP packet is routed across multiple networks.

This distinction is fundamental:

Layer 2 = local/hop-by-hop delivery

Layer 3 = routing between networks

34. Destination IP Consistency

Suppose:

PC → R1 → R2 → R3 → Server

Normally:

Destination IP:
Server IP → Server IP → Server IP → Server IP

The Layer-2 destination MAC changes at each hop.

However, there are technologies such as NAT that can modify IP addresses. NAT will be studied later.

Therefore, in a normal routing scenario:

The destination IP remains the ultimate destination's IP while the packet is being routed.

35. Frame Packet Segment

Segment:

Transport-layer unit.

TCP Header + Application Data

→ TCP Segment

Packet:

Network-layer unit.

IP Header + TCP Segment

→ IP Packet

Frame:

Data Link-layer unit.

Data Link Header + Packet + Trailer

→ Frame

Therefore:

Segment → Transport
Packet  → Network
Frame   → Data Link
Bits    → Physical

36. Complete Multi Hop Example

Consider:

PC → R1 → R2 → Server

Application creates:

HTTP Request

Transport:

TCP Header + HTTP
        ↓
TCP Segment

Network:

IP Header + TCP Segment
        ↓
IP Packet

Data Link:

Ethernet Header + IP Packet + Trailer
        ↓
Frame

Physical:

Frame
 ↓
Bits/signals
 ↓
R1

At R1:

Signals
 ↓
Frame
 ↓
Process Layer 2
 ↓
IP Packet
 ↓
Look at destination IP
 ↓
Routing table
 ↓
Select R2
 ↓
Create NEW Layer-2 frame
 ↓
Transmit

At R2:

New incoming frame
 ↓
Process Layer 2
 ↓
IP Packet
 ↓
Look at destination IP
 ↓
Routing table
 ↓
Select Server
 ↓
Create NEW Layer-2 frame
 ↓
Transmit

At Server:

Frame
 ↓
Data Link processing
 ↓
IP packet
 ↓
Network processing
 ↓
TCP segment
 ↓
Transport processing
 ↓
HTTP/application data

37. Important Interview Traps

Trap 1:

"A router uses MAC addresses to route packets."

❌ Incorrect.

Routing decisions are based primarily on the destination IP address.

MAC is used for local Layer-2 delivery.

Trap 2:

"A switch routes using IP."

❌ For a traditional Layer-2 switch.

It forwards frames using MAC addresses.

Layer-3 switches can perform routing.

Trap 3:

"The server's MAC address is placed in the first frame."

❌ Not necessarily.

The first frame is addressed to the next local-hop device, usually the default gateway/router when the destination is on another network.

Trap 4:

"The same Ethernet frame travels across the entire Internet."

❌ No.

A new Layer-2 frame is created for each hop/link.

Trap 5:

"The MAC address stays the same end-to-end."

❌ No.

The Layer-2 source/destination MAC addresses change from hop to hop.

Trap 6:

"The IP packet is completely removed and recreated from scratch at every router."

❌ Not in the same sense as Layer-2 framing.

The router processes and forwards the IP packet while creating a new Layer-2 frame for the outgoing link.

Some IP header fields can change during forwarding, such as TTL.

Trap 7:

"The Data Link layer removes the TCP header."

❌ No.

Each layer processes its own protocol information.

Data Link → Layer-2 header/trailer
Network   → IP header
Transport → TCP/UDP header

38. End To End Mental Model

The entire Level 1 can be summarized as:

APPLICATION
    │
    │ HTTP / DNS / etc.
    ↓
TRANSPORT
    │
    │ TCP / UDP + ports
    ↓
NETWORK / INTERNET
    │
    │ IP + routing
    ↓
DATA LINK / LINK
    │
    │ MAC + frames
    ↓
PHYSICAL
    │
    │ Bits / signals
    ↓
NETWORK

During sending:

Data
 ↓
Segment
 ↓
Packet
 ↓
Frame
 ↓
Bits

During receiving:

Bits
 ↓
Frame
 ↓
Packet
 ↓
Segment
 ↓
Data

39. Placement Ready Answers

What is the OSI model?

The OSI model is a seven-layer reference model that divides network communication into separate responsibilities: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

What is the TCP/IP model?

TCP/IP is the practical protocol architecture used by the Internet. It is commonly represented using four layers: Application, Transport, Internet, and Network Access/Link.

Difference between OSI and TCP/IP?

OSI is a seven-layer reference model with separate Session, Presentation, and Physical layers. TCP/IP is a practical Internet protocol architecture that commonly combines OSI Application, Presentation, and Session into Application, and Data Link and Physical into the Link/Network Access layer.

What is encapsulation?

Encapsulation is the process of adding protocol-specific control information as data moves down the networking layers. Application data becomes a segment, packet, frame, and finally bits/signals.

What is decapsulation?

Decapsulation is the reverse process at the receiver, where each layer processes the information belonging to its protocol and passes the remaining payload upward.

Why does a router use IP rather than MAC for routing?

IP is used for Layer-3 routing because the destination IP determines the destination network and the routing table determines the next hop/interface. MAC is then used for Layer-2 delivery over the selected local link.

Why does the MAC address change at every router?

Because the Layer-2 frame is hop-by-hop. Each router removes/processes the incoming Layer-2 frame and creates a new frame for the outgoing link with the appropriate source and destination MAC addresses.

40. Final Level 1 Mental Model

If you remember nothing else, remember this:

                     END-TO-END
                         │
                    Destination IP
                         │
                         ▼
PC ─────── R1 ─────── R2 ─────── Server
 │          │          │           │
 │          │          │           │
Frame 1   Frame 2    Frame 3     Frame 4
 │          │          │           │
 └──────────── Layer 2 ────────────┘
             HOP-BY-HOP

IP packet → routed across networks
Frame     → changes at every hop
MAC       → next-hop/local delivery
IP        → network-level routing
Port      → destination process
TCP       → transport-level communication
HTTP      → application-level communication

40.1 The Three Rules To Remember

1. IP decides where the packet should go.

2. MAC delivers the packet to the next hop on the current local link.

3. Each layer processes its own protocol's header.

And the fundamental data-unit chain:

Data → Segment → Packet → Frame → Bits

These concepts form the foundation for everything that follows in CN — especially switching, MAC addresses, ARP, IP addressing, routing, TCP, and the full browser-to-server request flow.