Lower Layers

Computer Networks - Lower Layers

The Underground Mail System of the Internet

Imagine you’re sending a birthday party invitation to your friend across town. You can’t just shout it — you need to write it on paper, put it in an envelope with an address, and give it to a mail carrier who knows all the streets. Computer networks work exactly like this magical postal system, but for digital messages!

The Big Picture: Layers Like a Cake

Think of network communication like a layer cake:

• Each layer has its own job
• They work together to deliver your message
• The lower layers handle the physical delivery (like the roads, trucks, and addresses)

graph TD
    A["Your Message"] --> B["IP Layer - Street Directions"]
    B --> C["Ethernet/MAC - Your Home Address"]
    C --> D["Physical Layer - The Actual Road"]
    D --> E["Destination Computer"]

1. Physical Layer - The Actual Roads and Wires

What Is It?

The physical layer is like the actual roads mail trucks drive on. In computers, it’s the cables, radio waves, and light signals that carry your data.

Simple Example

When you plug in an Ethernet cable, that cable IS the physical layer. The electricity or light traveling through it carries your message as tiny pulses — like cars zooming on a highway!

Key Concepts

Cables and Signals:

• Copper cables carry electrical pulses (0s and 1s as voltage changes)
• Fiber optic cables use light flashes (super fast!)
• Wireless uses radio waves (like invisible light)

Bits Are Like Morse Code:

High voltage = 1  (dot)
Low voltage = 0   (dash)

Hello = 01001000 01100101 01101100 01101100 01101111

Think of it as blinking a flashlight to spell words to your friend across the street!

2. Error Detection - Did My Letter Get Smudged?

What Is It?

Sometimes messages get damaged during travel. Maybe rain smudged the ink, or static electricity confused the signal. Error detection is like double-checking your letter arrived readable.

The Checksum Trick

Imagine you’re sending the numbers: 3, 5, 7

You add them up: 3 + 5 + 7 = 15

You send: 3, 5, 7, 15

Your friend receives them, adds 3+5+7, and checks if it equals 15. If not, something went wrong!

Real-World Methods

Parity Bit - The “buddy system”:

Data:    1 0 1 1 0 1 0  (count the 1s = 4, even!)
Parity:  0              (add 0 to keep it even)
Result:  1 0 1 1 0 1 0 0

CRC (Cyclic Redundancy Check) - A smarter checksum:

• Uses math division instead of simple addition
• Catches more errors
• Used in Ethernet, WiFi, and hard drives

graph TD
    A["Original Message"] --> B["Calculate Checksum"]
    B --> C["Send Message + Checksum"]
    C --> D["Receiver Calculates Own Checksum"]
    D --> E{Do They Match?}
    E -->|Yes| F["Message OK!"]
    E -->|No| G["Error Detected!"]

3. Error Correction - Fixing Smudged Letters Automatically

What Is It?

Error correction is magical — it doesn’t just FIND problems, it FIXES them without asking for a re-send!

The Hamming Code Story

Imagine you write each important word THREE times:

CAT CAT CAT

If one gets smudged to “CXT CAT CAT”, your friend sees two say CAT and knows the real word.

Real Hamming Code:

Original: 1 0 1 1
Encoded:  1 0 1 1 0 0 1  (extra bits added)

Those extra bits let computers vote on what each bit should be!

When Do We Use This?

• Satellites - Can’t easily ask for re-sends (too far away!)
• DVDs and CDs - Scratches happen, but music still plays
• RAM Memory - Critical data can’t wait

4. MAC Addressing - Your Computer’s Birth Certificate

What Is It?

A MAC (Media Access Control) address is like your fingerprint — unique to YOUR specific network card. No two in the world are the same!

What Does It Look Like?

AA:BB:CC:DD:EE:FF

• 6 pairs of hexadecimal numbers
• 48 bits total
• First 3 pairs = manufacturer
• Last 3 pairs = unique device number

Simple Example

Apple iPhone:    A4:83:E7:xx:xx:xx
Dell Computer:   00:14:22:xx:xx:xx

The first part tells you who made the device, like how your last name tells people your family!

Why Does It Matter?

• Your MAC address only works on your local network (like house numbers only work in your neighborhood)
• Routers use it to know which device to hand messages to
• It NEVER changes (unlike your IP address)

5. ARP Protocol - The Name Detective

What Is It?

ARP (Address Resolution Protocol) is like a detective that finds out “Hey, I have a letter for IP 192.168.1.5 — which computer has that address?”

How It Works (Story Time!)

graph TD
    A["Your Computer"] --> B["Shouts: Who has 192.168.1.5?"]
    B --> C["Everyone on network hears"]
    D["Target Computer"] --> E[Replies: That's me! My MAC is AA:BB:CC:DD:EE:FF]
    E --> F["Your Computer"]
    F --> G["Now can send directly!"]

Step by Step:

1. You want to send to IP 192.168.1.5
2. You don’t know their MAC address
3. You broadcast: “Who has 192.168.1.5?”
4. That computer replies with its MAC
5. You remember this (cache it) for later!

Real Example

Your PC:    Who has 192.168.1.1? Tell 192.168.1.100
Router:     192.168.1.1 is at AA:11:22:33:44:55

6. Ethernet - The Language of Local Networks

What Is It?

Ethernet is the rules of the road for your local network. It defines how devices talk on the same network, take turns, and format their messages.

Ethernet Frame (The Envelope)

┌──────────────────────────────────────────────────┐
│ Destination MAC │ Source MAC │ Type │ DATA │ CRC │
│   (6 bytes)     │ (6 bytes)  │(2 B) │  ... │(4 B)│
└──────────────────────────────────────────────────┘

Think of it like an envelope:

• Destination MAC = Where it’s going
• Source MAC = Who sent it
• Type = What language is inside (IPv4? IPv6?)
• DATA = Your actual letter
• CRC = Error check stamp

Speeds Through History

7. IP Addressing - Your Computer’s Street Address

What Is It?

While MAC is your fingerprint, IP address is like your street address — it tells the network WHERE to find you.

IPv4 Format

192.168.1.100

• 4 numbers (0-255 each)
• Separated by dots
• 32 bits total
• About 4.3 billion possible addresses

Public vs Private (Home vs World)

Private (Your Home Network):
  192.168.x.x
  10.x.x.x
  172.16-31.x.x

Public (The Internet):
  Everything else!
  Like: 8.8.8.8 (Google's DNS)

Think of it this way:

• Private IP = Your room number in your house
• Public IP = Your actual street address

8. IPv6 Basics - When We Ran Out of Addresses!

The Problem

IPv4 has only 4.3 billion addresses. With phones, laptops, smart fridges, and even light bulbs needing addresses — we ran out!

The Solution: IPv6

2001:0db8:85a3:0000:0000:8a2e:0370:7334

• 8 groups of 4 hexadecimal characters
• 128 bits total
• Enough for 340 undecillion addresses!

Simplified Writing

Full:    2001:0db8:0000:0000:0000:0000:0000:0001
Short:   2001:db8::1

• Remove leading zeros
• Replace consecutive zero groups with ::

Comparison

9. Subnetting - Dividing Your Neighborhood

What Is It?

Subnetting is like dividing a big apartment building into floors. Each floor (subnet) has its own group of apartments (IP addresses).

Why Do It?

• Organization - Keep departments separate
• Security - Limit who can see what
• Performance - Less traffic noise

The Subnet Mask

IP Address:     192.168.1.100
Subnet Mask:    255.255.255.0

The mask tells you which part is the network (street) and which part is the host (house number):

IP:    192.168.1   .   100
       └─Network─┘    └Host┘

CIDR Notation (Shorthand)

192.168.1.0/24

The /24 means:
- First 24 bits = network portion
- Remaining 8 bits = host addresses
- 256 possible addresses (254 usable)

Common Subnet Masks

10. Routing Fundamentals - The GPS of the Internet

What Is It?

Routing is how data finds its way from your computer to a server across the world. Routers are like GPS navigators at every intersection, pointing your data in the right direction.

How Routers Decide

Each router has a routing table — like a map book:

┌─────────────────────────────────────────────┐
│ Destination     │ Next Hop      │ Interface │
├─────────────────────────────────────────────┤
│ 192.168.1.0/24  │ Directly      │ eth0      │
│ 10.0.0.0/8      │ 192.168.1.1   │ eth0      │
│ 0.0.0.0/0       │ 192.168.1.254 │ eth0      │
└─────────────────────────────────────────────┘

• Destination = Where are you trying to go?
• Next Hop = Who to ask next
• Default route (0.0.0.0) = “If I don’t know, ask this guy”

The Journey

graph LR
    A["Your PC"] --> B["Home Router"]
    B --> C["ISP Router"]
    C --> D["Internet Backbone"]
    D --> E[Google's Router]
    E --> F["Google Server"]

Each step is a “hop”. A message might take 10-20 hops to cross the world!

Types of Routing

Static Routing:

• Humans manually set the paths
• Simple but doesn’t adapt to changes
• Good for small networks

Dynamic Routing:

• Routers talk to each other
• Automatically find best paths
• Adapts when links fail

Putting It All Together

When you visit google.com:

1. Physical Layer - Your WiFi card sends radio waves to the router
2. Ethernet - Message wrapped with MAC addresses for local delivery
3. ARP - Finds the router’s MAC address
4. IP - Your message has Google’s IP as destination
5. Routing - Each router passes your message closer to Google
6. Error Detection - Every hop checks for damage

It all happens in milliseconds!

Remember These Key Ideas

• Physical Layer = The actual wires and signals (roads)
• Error Detection = Double-checking messages arrived intact
• Error Correction = Auto-fixing damaged data
• MAC Address = Unique device fingerprint (never changes)
• ARP = Detective that maps IP to MAC addresses
• Ethernet = Rules for local network communication
• IP Address = Your location on the internet
• IPv6 = New addressing system (we ran out of IPv4!)
• Subnetting = Dividing networks into smaller groups
• Routing = GPS system that guides data across networks

You now understand the underground mail system of the internet! Every time you load a webpage, all these layers work together like a perfectly choreographed dance. Pretty amazing, right?