IP Addressing

Classful Addressing (Class A: 10.0.0.0/8)

Set up a basic LAN in GNS3 using a hub and four VPCS nodes. Use Class A private IP addresses (10.0.0.0/8), assign static IPs, test connectivity, and observe ARP/ICMP behavior.

• All PCs (VPCS1 to VPCS4) should communicate via the hub or switch. • Each PC should reply to ping from other PCs. • ARP tables should show MAC mappings after pinging. • Wireshark should capture ARP and ICMP packets.

Steps:

1. Open GNS3 → create a new project (e.g., LAN_Hub_Test).

2. Add Devices

   • Search for and add a Hub to the workspace.

   • Add 4 VPCS nodes (name them VPCS1 to VPCS4).

3. Connect the Devices

   • Use the Add a Link tool (cable icon).

   • Connect each VPCS’s Ethernet0 interface to a separate port on the hub.

4. Assign IP Addresses (Class A: 10.0.0.0/8)
   Open each VPCS console and enter:

ip 10.0.0.1 255.0.0.0    → VPCS1

ip 10.0.0.2 255.0.0.0    → VPCS2

ip 10.0.0.3 255.0.0.0    → VPCS3

ip 10.0.0.4 255.0.0.0    → VPCS4

save     → On each VPCS after assigning IP

5. Test Network Connectivity
   From VPCS1, run:

ping 10.0.0.2
ping 10.0.0.3
ping 10.0.0.4

• Each ping should return replies → means successful communication.

6. Check ARP Table on VPCs (optional)
   • You should see IP-to-MAC mappings of other PCs.

show arp

Understanding What’s Happening

• When pinging a device for the first time: An ARP Request is sent to find the destination MAC. The target replies with an ARP Reply. This MAC address is stored in the ARP table.

• ICMP Echo Request and Echo Reply are what you see during a ping.

• TTL (Time To Live) field in the IP header indicates the number of routes (hops) a packet has taken. If TTL expires before reaching the destination then packet is dropped. This helps avoid endless loops:

  • Windows: 128, Linux: 64, Routers: 255

Optional – Capture Traffic with Wireshark

• Right-click a link → Start capture with Wireshark.

• You’ll see ARP and ICMP packets when you ping.

Extra Practice

• Replace Hub with a Switch → compare behavior.

• Try same setup with:

  • Class B: 172.16.0.0/16

  • Class C: 192.168.1.0/24

• Notice differences in IP range and subnet masks.

Outcome

• All PCs must be on the same subnet for direct communication.

• Hub broadcasts traffic to all nodes → good for observing ARP behavior.

• Switches only forward to intended recipients (MAC-based filtering).