🌐 Computer Networks Lab — Session 1
1. Network Topology: Star Topology
🧩 What Is a Network Topology?
A network topology defines the layout or structure of how devices (computers, routers, switches, etc.) are connected in a network.
⭐ Star Topology
In a star topology, all devices are connected to a central device (like a switch or hub). This central device manages and controls all communication between devices.
🖼️ Visual Representation
[PC1]
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[PC2] ---- [Switch/Hub] ---- [PC3]
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[PC4]
⚙️ Characteristics
- Each device has a dedicated connection to the central hub.
- If one connection fails, only that device is affected.
- The hub or switch acts as the central communication point.
✅ Advantages
- Easy to install and manage.
- Easy to detect faults or remove devices.
❌ Disadvantages
- If the central hub fails, the whole network goes down.
- Requires more cabling than some other topologies.
2. DNS (Domain Name System)
🧠 Concept
DNS stands for Domain Name System. It translates human-friendly domain names (like google.com) into machine-friendly IP addresses (like 8.8.8.8).
⚙️ How It Works
When you type www.yahoo.com in your browser:
- Your computer asks a DNS server for the IP of that domain.
- The DNS server replies with the IP address (e.g.,
74.6.231.20). - Your computer connects to that IP — not the domain name.
💡 Analogy
Think of DNS as the Internet’s phonebook — it maps names to numbers.
3. IP Addresses
🧩 What Is an IP Address?
An IP address (Internet Protocol address) is a unique identifier for each device connected to a network.
It’s like a home address for your device — it tells where data should go and where it came from.
🧮 IPv4 (Internet Protocol Version 4)
- Format: Four numbers (0–255), separated by dots — e.g.
192.168.1.1 - Example Range:
0.0.0.0to255.255.255.255 - Total possible addresses: ≈ 4.3 billion
- Problem: IPv4 addresses are running out due to the massive number of devices.
🧮 IPv6 (Internet Protocol Version 6)
- Format: Eight groups of four hexadecimal digits — e.g.
2001:0db8:85a3:0000:0000:8a2e:0370:7334 - Total possible addresses: ≈ 340 undecillion (a number with 36 zeros!)
- Designed to handle the explosion of Internet-connected devices (IoT, sensors, smart devices).
🧱 IP Address Classes (IPv4)
IPv4 is divided into five classes based on range and usage.
| Class | Range of First Octet | Example IP | Default Subnet Mask | Use Type |
|---|---|---|---|---|
| A | 1 – 126 | 10.0.0.1 | 255.0.0.0 | Very large networks |
| B | 128 – 191 | 172.16.0.1 | 255.255.0.0 | Medium networks |
| C | 192 – 223 | 192.168.1.1 | 255.255.255.0 | Small networks |
| D | 224 – 239 | 224.0.0.1 | — | Multicasting |
| E | 240 – 255 | — | — | Experimental |
🌍 Private IP Ranges (Not Publicly Routable)
Only three IPv4 ranges are reserved for private networks (LANs):
| Class | Private Range | Example Use Case |
|---|---|---|
| A | 10.0.0.0 – 10.255.255.255 | Large private LANs |
| B | 172.16.0.0 – 172.31.255.255 | Medium private LANs |
| C | 192.168.0.0 – 192.168.255.255 | Home/office networks |
These IPs are free to use locally but not reachable on the Internet.
4. Subnet Mask and VLSM
🧩 Subnet Mask
A subnet mask defines which part of an IP address refers to the network and which part refers to the host (the device).
Example: IP Address: 192.168.1.10 Subnet Mask: 255.255.255.0
This means:
192.168.1→ Network portion.10→ Host portion
So, this network can have 254 hosts (1–254).
🧮 VLSM (Variable Length Subnet Mask)
VLSM allows dividing a network into sub-networks of different sizes, improving IP address efficiency.
Example:
- Instead of giving every department a
/24(256 addresses), you can give:/26for a 50-device department/28for a 10-device department
5. Types of Communication
| Type | Description | Example |
|---|---|---|
| Unicast | One sender → One receiver | A PC sends a file to another PC |
| Multicast | One sender → Selected group | Streaming video to specific subscribers |
| Broadcast | One sender → All devices in network | Sending an ARP request |
6. MAC Address (Media Access Control)
🧩 Definition
A MAC address is a unique hardware identifier assigned to a network interface card (NIC).
- It is burned into the hardware.
- It works at the Data Link Layer (Layer 2).
- Format: 6 pairs of hexadecimal numbers
Example:00:1A:2B:3C:4D:5E
⚙️ Difference Between IP and MAC
| Property | IP Address | MAC Address | |:———-|:————|:————-| | Type | Logical (can change) | Physical (fixed) | | Layer | Network Layer | Data Link Layer | | Assigned By | ISP or network admin | Device manufacturer | | Example | 192.168.1.10 | 00:1A:2B:3C:4D:5E |
7. Static vs Dynamic IP
| Type | Description | Assigned By | Example Use |
|---|---|---|---|
| Static IP | Permanently assigned; doesn’t change | Manually set or bought | Servers, domains |
| Dynamic IP | Temporarily assigned; changes over time | DHCP (usually your ISP) | Home Internet connections |
⚙️ How Dynamic IP Works
- When you connect your modem or router to your ISP:
- It sends a request to the DHCP server.
- The DHCP server assigns a temporary IP (lease).
- When you disconnect or after some time, it may change.
So, your home network shares one public IP from the ISP, while each device at home gets a local private IP (e.g., 192.168.x.x).
Your router uses NAT (Network Address Translation) to connect all your local devices to the Internet through that one public IP.
8. IPv6 and IoT (Internet of Things)
🧩 Why IPv6?
IPv4 can’t provide enough unique addresses for billions of new IoT devices — smart homes, cars, sensors, etc. IPv6 solves this by offering vast address space.
🌾 IoT Example
A smart farm with:
- Soil sensors
- Drones
- Smart irrigation
- Weather monitors
Each device may need a unique IP to connect to the cloud directly — that’s why IPv6 is crucial.
9. Local Networks and Global Internet
🌐 Local Network
You can assign any private IPs locally (e.g., 192.168.1.x), but those are not globally unique — they only work inside your LAN.
☁️ Global Connectivity
If a device needs to connect to the Internet (cloud) directly, it must have a public IP that is unique worldwide.
That’s why your idea of one IP per modem works for local communication, but not for global Internet access — only the modem/router has the public IP, and internal devices share it via NAT.
🧠 Summary
| Concept | Description |
|---|---|
| Star Topology | All devices connect to a central hub/switch |
| DNS | Translates domain names to IP addresses |
| IP Address | Unique identifier for a device |
| IPv4 vs IPv6 | IPv6 created to solve IPv4 address exhaustion |
| Subnet Mask | Separates network and host parts of an IP |
| VLSM | Efficient subnetting technique |
| Unicast / Multicast / Broadcast | Different message delivery types |
| MAC Address | Unique hardware identifier |
| Static/Dynamic IP | Permanent vs temporary address assignment |
| NAT | Allows multiple private devices to share one public IP |
| IPv6 & IoT | Provides massive address space for connected devices |