🧪 Basic Network Setup & Connectivity Lab

📌 Objective

Understand how IP addressing, subnetting, and basic connectivity work in a controlled virtual environment.

⚙️ Environment

  • Virtualization: VirtualBox
  • OS: Ubuntu Server (2 VMs)
  • Network mode: Internal Network + NAT

VMs

VM IP Address Role
VM1 192.168.10.10/24 Client
VM2 192.168.10.20/24 Server

🛠️ Setup

Step 1 - Create Virtual Machines

Create two Linux virtual machines:

  • server_01
  • server_02

Recommended specs:

  • 1 CPU
  • 1-2 GB RAM
  • 10+ GB disk

Step 2 - Configure Network

Both VMs must use the same Internal Network This ensures both machines are on the same isolated Layer 2 network.

Step 3 - Assign Static IPs

server_01 and server_02 configuration

Edit netplan:

sudo nano /etc/netplan/01-netcfg.yaml
sudo netplan apply

Netplan Netplan

Step 4 - Verify configuration

Check ip addresses:

ip a

Expectet output:

  • server_01 -> 192.168.10.10
  • server_02 -> 192.168.10.20

Server1_IP Server2_IP

Step 5 - Connectivity Test

From server_01:

ping 192.168.10.20

From server_02:

ping 192.168.10.10

Expected result:

  • Successful ICMP replies
  • Direct communication (same subnet)

Ping_srv1 Ping_srv2

Step 6 - Traceroute Test

Install traceroute:

sudo apt install traceroute

Traceroute test:

traceroute 192.168.10.20

Expected result:

  • Single hop (direct connection)
  • No routes involved

Traceroute

🧪 Experiment - Break the Network (Subnet Mismatch)

Change server_02 IP

addresses: 
- 192.168.20.20/24

sudo netplan apply

🧠 Debugging

  1. Confirm local network configuration

On server_01:

ip a

Expected result:

  • Correct IP assigned (192.168.10.10/24)
  • Correct subnet mask (/24)

Server_01_IP

  1. Verify routing table
ip route

Key observation:

  • Only local subnet route exists
  • No route to 192.168.20.0/24

IP_ROUTE

  1. Check target host reachability assumption

Verify server_02 configuration:

ip a

Wrong_IP

Key observation:

  • Target IP is valid (192.168.20.20/24)
  • server_02 belongs to a different subnet
  1. Analyze packet decision process

When server_01 send traffic:

ping 192.168.20.20

Ping

Linux checks routing table:

  • Is 192.168.20.20 in 192.168.10.0/24 ? -> ❌ No
  • Use default gateway
  1. Validate gateway capability

Check assumption:

  • Default gateway is NAT (VirtualBox)
  • It has no route to internal lab networks

Result: ❌ Gateway cannot reach 192.168.20.0/24

  1. Confirm absence of router between the subnets

Check environment design:

  • Only two VMs exist
  • No routing device configured
  • No IP forwarding enabled

Conclusion: 👉 No Layer 3 device connects both networks

🚨 Root Cause

The issue is:

❌ Subnet mismatch with no routing infrastructure

Specifically:

  • server_01: 192.168.10.0/24
  • server_02: 192.168.20.0/24
  • No router exists between them

🔧 Resolution

Restore same subnet on server_02:

addresses:
 - 192.168.10.20/24

sudo netplan apply

🧪 Validation

from server_01:

ping 192.168.10.20

Ping_srv1

🧠 Key Takeaways

  • Devices must be in the same subnet for direct communication
  • Routing is required between different networks
  • Linux always checks routing table before sending packets
  • Default gateway is only used when no local route exists

🧭 Mental Model

When troubleshooting connectivity:

  1. Is IP correct?
  2. Is subnet correct?
  3. Is destination in routing table?
  4. Is a router required?
  5. Does the router actually exists?