Network Config: IP, Routing, Subnetting, Topologies & Protocols

TAKEAWAYS:

• Understand IPv4/IPv6 addressing, subnetting, and CIDR notation to design and troubleshoot networks.
• Differentiate static vs. dynamic routing and know when to use each (e.g., small networks vs. ISPs).
• Visualize network topologies (star, mesh, bus) and their pros/cons for real-world deployments (e.g., NTC’s backbone vs. a home Wi-Fi).
• Master TCP/IP protocol stack layers and how protocols like ICMP, ARP, and DNS enable communication.
• Configure Linux/Windows interfaces (IP, subnet mask, gateway) and verify connectivity using ping, traceroute, and ip route.
• Apply subnetting rules to divide a network into subnets (e.g., splitting a class C address for a university’s departments).

1. IP Addressing and Subnetting: The Foundation of Network Communication

Every device on a network needs a unique identifier: an IP address. IPv4 uses 32-bit addresses (e.g., 192.168.1.1), while IPv6 uses 128-bit addresses (e.g., 2001:0db8::1). Subnetting divides a network into smaller subnets to improve efficiency and security.

1.1 IP Address Classes (Legacy)

IPv4 addresses are divided into classes based on the first octet:

Legacy IPv4 classful addressing ranges (Image: Michel Bakni, CC BY-SA 4.0, via Wikimedia Commons)

1.2 Subnetting: Dividing Networks Efficiently

Subnetting splits a network into smaller subnets using a subnet mask. The subnet mask defines which bits are for the network portion and which are for the host portion.

Example: Subnetting a Class C Network (192.168.1.0/24)

• Given: Network 192.168.1.0 with subnet mask 255.255.255.0 (8 host bits).
• Goal: Create 4 subnets with at least 14 hosts each.
• Steps:
  1. Borrows bits: Need 2 bits for 4 subnets (2^2 = 4).
  2. New subnet mask: 255.255.255.192 (26 bits, /26).
  3. Subnet IDs:
     • 192.168.1.0 (00)
     • 192.168.1.64 (01)
     • 192.168.1.128 (10)
     • 192.168.1.192 (11)
  4. Host ranges:
     • Subnet 1: 192.168.1.1–192.168.1.62
     • Subnet 2: 192.168.1.65–192.168.1.126
     • (Repeat for others.)

Worked Example: NTC’s Backbone Network NTC uses a /16 network (10.0.0.0/16) for its backbone. To connect 5 regional offices, they subnet it into /24 subnets:

• Subnet 1: 10.0.1.0/24 (Kathmandu)
• Subnet 2: 10.0.2.0/24 (Pokhara)
• ...
• Subnet 5: 10.0.5.0/24 (Biratnagar)

1.3 CIDR Notation and Variable-Length Subnet Masking (VLSM)

CIDR (Classless Inter-Domain Routing) uses slash notation (e.g., /24) to represent subnet masks. VLSM allows flexible subnet sizing based on host needs.

Example: VLSM for a University Network

Mermaid Diagram: CIDR Subnetting

mindmap
  root((192.168.1.0/24))
    CIDR Subnetting
      /27 (30 hosts)
        192.168.1.0/27
      /28 (14 hosts)
        192.168.1.32/28
        192.168.1.48/28
      /26 (62 hosts)
        192.168.1.64/26

2. Network Topologies: How Devices Are Connected

Topology defines how devices (computers, routers, switches) are physically or logically connected. The choice affects scalability, cost, and fault tolerance.

2.1 Common Topologies

Real-World Example: Pathao’s Delivery Network Pathao uses a hybrid topology:

• Star topology for rider-to-server communication (each rider’s phone connects to a central server).
• Mesh topology for backup routes if a server fails (redundancy).

3. Routing: How Data Finds Its Way

Routing determines the path packets take from source to destination. Routers use routing tables to make decisions.

3.1 Static vs. Dynamic Routing

Example: Configuring Static Routes in Linux

# Add a static route to 192.168.2.0/24 via gateway 192.168.1.2
sudo ip route add 192.168.2.0/24 via 192.168.1.2

# Make it persistent (add to /etc/network/interfaces or netplan)
echo "192.168.2.0/24 via 192.168.1.2" | sudo tee -a /etc/network/interfaces

Mermaid Diagram: Static Routing Example

graph TD
    A["PC1: 192.168.1.10"] -->|"Destination: 192.168.2.50"| B["Router1: 192.168.1.1"]
    B -->|"Route: 192.168.2.0/24 via 192.168.1.2"| C["Router2: 192.168.1.2"]
    C --> D["PC2: 192.168.2.50"]

3.2 Dynamic Routing Protocols

Example: OSPF in NTC’s Network NTC uses OSPF to dynamically route traffic between regional offices. If a link fails (e.g., Kathmandu-Pokhara fiber cut), OSPF recalculates routes in seconds.

4. TCP/IP Protocol Suite: The Rules of Network Communication

The TCP/IP model has 4 layers (vs. OSI’s 7). Each layer has protocols for specific functions.

Mermaid Diagram: TCP/IP vs. OSI Layers

flowchart TD
    subgraph OSI Model
        A["Application"] --> B["Presentation"] --> C["Session"] --> D["Transport"] --> E["Network"] --> F["Data Link"] --> G["Physical"]
    end
    subgraph TCP/IP Model
        H["Application"] --> I["Transport"] --> J["Internet"] --> K["Network Access"]
    end

4.1 Key Protocols by Layer

Worked Example: How YouTube Uses TCP/IP

1. Application Layer: You type youtube.com → DNS resolves it to 142.250.190.46.
2. Transport Layer: Your browser uses TCP to establish a connection (3-way handshake).
3. Internet Layer: IP packets are routed via ISPs (e.g., Ncell → Google’s CDN).
4. Network Access: Ethernet/Wi-Fi transmits frames to your device.

SYN, SYN-ACK, ACK process (Image: CC BY-SA 3.0, via Wikimedia Commons)

5. Configuring Network Interfaces in Linux

Linux uses commands like ip, ifconfig (legacy), and /etc/network/interfaces to configure interfaces.

5.1 Basic Commands

Example: Configuring a Linux Server for a Web Host

# Assign IP and enable interface
sudo ip addr add 203.127.199.100/24 dev ens33
sudo ip link set ens33 up

# Set default gateway
sudo ip route add default via 203.127.199.1

# Test connectivity
ping 203.127.199.1

6. Troubleshooting Network Issues

Common issues and commands to diagnose them:

Example: Fixing a Subnet Mismatch

• Problem: Two PCs on 192.168.1.0/24 can’t ping each other.
• Diagnosis:

  PC1: ip a → 192.168.1.10/25
  PC2: ip a → 192.168.1.200/25
  

• Solution: Both must be on the same subnet (e.g., /24). Change PC2’s subnet mask to 255.255.255.0.

In the Real World

1. eSewa and Khalti (Payment Gateways)

   • Idea Used: Load balancing and dynamic routing.
   • How: During Diwali, millions of transactions hit eSewa’s servers. They use BGP for dynamic routing to distribute traffic across multiple data centers (e.g., Kathmandu, Pokhara). If one server fails, BGP reroutes traffic instantly.

2. Daraz (E-Commerce)

   • Idea Used: Subnetting and VLSM.
   • How: Daraz’s backend uses /24 subnets for different services (e.g., 10.0.1.0/24 for web servers, 10.0.2.0/24 for databases). VLSM ensures they don’t waste IPs on small departments (e.g., HR gets /28).

3. NTC’s Fiber Optic Backbone

   • Idea Used: Mesh topology and OSPF.
   • How: NTC’s national fiber network uses a partial mesh for redundancy. If a fiber link fails (e.g., Kathmandu-Pokhara), OSPF recalculates routes in <10 seconds, ensuring no downtime for Ncell or Internet users.

Exam Tip

1. Subnetting is a high-weight topic: Always show your work step-by-step (e.g., "borrow 2 bits for 4 subnets").
2. Compare static vs. dynamic routing: Know when to use each (e.g., static for home routers, dynamic for ISPs).
3. TCP/IP layers: Memorize protocols by layer (e.g., HTTP is Application, ARP is Network Access).
4. Real-world scenarios: Expect questions like:
   • "A university has 5 departments. Design a subnet plan for 100, 50, 30, 20, and 10 hosts."
   • "Explain how Pathao uses dynamic routing to handle rider locations."
5. Commands: Know ip, ping, traceroute, and nslookup outputs.
6. Diagrams: Draw OSI/TCP/IP layers, subnetting examples, and topologies in exams. Label everything!