W1·A1 — Two nodes, one cable

Time: 20 min

Needs: 2 VPCS and a cable

GNS3 project: w01-a1-two-nodes

Predict first

Write these down before touching GNS3.

1. Can two computers talk with just a cable running between them?
2. What other property, if at all, do they require before they can communicate?

Do this — build it

1. File → New blank project, name it w01-a1-two-nodes.
2. Browse End Devices → drag VPCS onto the canvas twice.
3. Click Add a link (the cable icon at the bottom of the device panel). Click PC1 → Ethernet0 → click PC2 → Ethernet0. Press Esc to put the cable tool away.
4. Clidk the green ▶ in the top toolbar to start both.
5. Right-click each → Console. You should get a PC1> (and PC2>) prompt immediately — no login required.

Congratulations! You've got yourself your first network!

Now, let's see if the two nodes can actually talk.

Do this — make them talk

# on PC1
show ip
ping 192.168.10.100

show ip reports no address. The ping fails. Now:

# on PC1
ip 192.168.10.100/24
show ip

# on PC2
ip 192.168.10.200/24
show ip

# back on PC1
ping 192.168.10.200
save

What just happened

• The cable was there the whole time and carried nothing. A link is not a network — it is one of the two things a network needs, and the other is addressing.
• Nothing was configured on the link. Both hosts were configured, independently, and they agreed by accident of you typing compatible numbers. Nobody negotiated anything.
• show ip also printed a MAC address. That one came with the interface.
  • We will talk about layers in a moment, but the presence of these two addresses is the first hint of that.
• save writes the config so a stop/start does not lose it.

Do this — break it without touching the cable

Both hosts still cabled, still working. On PC1 only:

ip 192.168.10.100/25
show ip

Still from PC1:

ping 192.168.10.200

What just happened

• It fails. You didn't take the cable away, and both hosts carry addresses, yet PC2 didn't answer
• PC1 reports not finding a gateway. Why?
• /25 is the culprit, which will make sense once we've covered IPv4. For now, interpret it as the two hosts "thinking" they are in two distinct networks, despite still being connected by a cable.

Do this - capture the conversation

• Right-click on the cable and press Start capture
• Press Ok on the dialog that pops up
• Run the ping again
• Does Wireshark show any activity?

What just happened

• The ping fails again
• Wireshark doesn't show any activity. PC1 put nothing on the wire
• Cables don't care about addresses. Hosts do.

Put it back:

# on PC1
ip 192.168.10.100/24
save

Record

If it went wrong