🔗 Session 3 — Connecting Two LANs Using a Router

🗓️ Overview

In this session, we extended our previous lab setup in Cisco Packet Tracer to connect two separate LANs using a router. This introduced us to the concept of inter-network communication and how routers forward data between different subnets.

We also learned how to manually configure router interfaces using the Command-Line Interface (CLI).

🔌 Part 1 — Network Topology

🖊️ Description

We began with two separate LANs:

LAN 1: 3 PCs connected to Switch 1, using IPs in the range 192.168.1.0/24.
LAN 2: 3 PCs connected to Switch 2, using IPs in the range 192.168.2.0/24.

Each LAN was then connected to a Router via FastEthernet interfaces:

FastEthernet0/0 connected to Switch 1.
FastEthernet0/1 connected to Switch 2.

⛰️ ASCII Topology Diagram

           +---------------------+
           |       Router        |
           |---------------------|
           | Fa1/0   |   Fa2/0   |
           |192.168.1.1  192.168.2.1|
           +-----------+-----------+
                       |           |
                +------+           +------+
                |                         |
         +-------------+           +-------------+
         |   Switch 1  |           |   Switch 2  |
         +------+------+           +------+------+
                |                         |
     192.168.1.0/24              192.168.2.0/24
     (3 PCs)                     (3 PCs)

💡 Tip: Each LAN must have a unique network ID. Routers connect different networks, so their connected interfaces must belong to different subnets.

🧮 Part 2 — Configuring the Router (CLI)

The router starts with no active interfaces. We configured two FastEthernet ports, one for each LAN.

⚙️ Step-by-Step Configuration

Router> enable                       # Enter privileged mode
Router# configure terminal            # Enter global configuration mode
Router(config)# interface fastethernet 1/0  # Access interface Fa1/0
Router(config-if)# ip address 192.168.1.1 255.255.255.0   # Assign IP for LAN 1
Router(config-if)# no shutdown                     # Enable the interface
Router(config-if)# exit
Router(config)# interface fastethernet 2/0          # Access interface Fa2/0
Router(config-if)# ip address 192.168.2.1 255.255.255.0   # Assign IP for LAN 2
Router(config-if)# no shutdown                     # Enable the interface
Router(config-if)# exit
Router(config)# exit
Router#

💡 Remember: The no shutdown command is crucial. Without it, the router interface remains in a down state (orange light in Packet Tracer).

🔌 Part 3 — Configuring the PCs

🔧 LAN 1 PCs

Device	IP Address	Subnet Mask	Default Gateway
PC1	192.168.1.2	255.255.255.0	192.168.1.1
PC2	192.168.1.3	255.255.255.0	192.168.1.1
PC3	192.168.1.4	255.255.255.0	192.168.1.1

🔧 LAN 2 PCs

Device	IP Address	Subnet Mask	Default Gateway
PC4	192.168.2.2	255.255.255.0	192.168.2.1
PC5	192.168.2.3	255.255.255.0	192.168.2.1
PC6	192.168.2.4	255.255.255.0	192.168.2.1

💡 Gateway: This is the IP address of the router interface connected to the same LAN. It acts as the exit point for all traffic leaving the LAN.

🛠️ Part 4 — Testing the Setup

Once all interfaces and PCs were configured:

Wait for the link lights on the router and switches to turn green (indicating the interfaces are up).
From a PC in LAN 1, open the Command Prompt and ping a PC in LAN 2.
```
ping 192.168.2.2
```
If configured correctly, you should receive successful replies — proving that the router is forwarding packets between the two networks.

🧠 Concepts Learned

Routers connect different networks and make inter-network communication possible.
Each LAN must have a unique IP network.
Router interfaces act as gateways for each subnet.
no shutdown activates an interface that is administratively down.
Packet Tracer LED colors:
- Orange = interface detected but inactive
- Green = interface active and operational

📚 Exam Tips

Never reuse IP addresses between different LANs.
The first usable IP (x.x.x.1) is often given to the router (gateway).
If ping fails, verify:
- The correct gateway is set on each PC.
- Router interfaces are not shut down.
- Each LAN uses the correct subnet mask.
Routers work at OSI Layer 3, using IP addresses.
Switches work at OSI Layer 2, using MAC addresses.

🏁 Summary

In this lab, we:

Connected two LANs through a router.
Assigned IP addresses and subnet masks to all devices.
Configured router interfaces with the CLI.
Verified connectivity using ping.
Understood the role of routers in interconnecting networks.