Objective

Same-network communication (Layer 2 only)
Different-network communication (Layer 3 + Layer 2)
When ARP is used
When the router is required
What changes and what stays constant
This is designed for beginners.

Devices

• 1 Router (R1)
• 1 Switch (SW1)
• 2 Hosts (Host-A and Host-B)

Physical Connections

• Host-A (link to) Switch
• Host-B (link to) Switch
• Router G0/0 (link to) Switch
• Router G0/1 (link to) Switch

No VLANs.
No trunking.
Just basic Ethernet.

IP Addressing

Device	Interface	IP Address	Subnet Mask	Default Gateway
Host-A	NIC	192.168.1.10	255.255.255.0	192.168.1.1
Host-B	NIC	192.168.2.20	255.255.255.0	192.168.2.1
R1	G0/0	192.168.1.1	255.255.255.0	—
R1	G0/1	192.168.2.1	255.255.255.0	—

You will use this same topology to test both scenarios.
* IPs are not configured on switches.

Configure the Router

On R1:

enable
configure terminal

interface g0/0
 ip address 192.168.1.1 255.255.255.0
 no shutdown
exit

interface g0/1
 ip address 192.168.2.1 255.255.255.0
 no shutdown
exit

end

Verify:

show ip interface brief
show ip route

You should see two connected routes:

C 192.168.1.0/24
C 192.168.2.0/24

Configure the Hosts

Host-A

IP: 192.168.1.10
Mask: 255.255.255.0
Gateway: 192.168.1.1

Host-B

IP: 192.168.2.20
Mask: 255.255.255.0
Gateway: 192.168.2.1

Test Different Network (Layer 3 Required)

From Host-A:

ping 192.168.2.20

It should work.

What Happens Internally

1. Host-A checks:
   • Is 192.168.2.20 in my network?
2. It calculates network ID.
3. Networks are different.
4. Host-A ARPs for default gateway (192.168.1.1).
5. Router replies with its MAC.
6. Host-A builds frame:
   • Destination MAC = Router
   • Source MAC = Host-A
   • IP addresses unchanged.
7. Router:
   • Removes L2 header
   • Decrements TTL
   • Looks at routing table
   • ARPs for Host-B
   • Builds new frame with new MAC addresses.
8. Host-B receives packet.

Important:

• IP addresses stay the same.
• MAC addresses change at router.
• TTL decreases.

Convert to Same Network (Layer 2 Only)

Now change only Host-B:

Change Host-B:

IP: 192.168.1.20
Mask: 255.255.255.0
Gateway: (leave blank or 192.168.1.1)

Now both Hosts are in:

192.168.1.0/24

Test again from Host-A:

ping 192.168.1.20

What Happens Now

1. Host-A checks destination network.
2. Network matches.
3. Host-A ARPs directly for Host-B.
4. Switch forwards frame.
5. Router is NOT involved.

Notice:

• Router G0/1 is unused.
• TTL does not decrement.
• No routing table lookup.
• No MAC rewrite at router.

Compare Both Results

Feature	Same Network	Different Network
Router Used	No	Yes
ARP For	Target host	Default gateway
TTL Decrement	No	Yes
MAC Changes at Router	No	Yes
IP Changes	No	No

Troubleshooting

Remove Host-A default gateway and try:

ping 192.168.2.20

It will fail.

Reason:
Host-A knows the network is remote but has no gateway to send traffic to.

Conclusion

Using a single topology, you observed both behaviors:

• Same subnet = pure Layer 2 delivery.
• Different subnet = Layer 3 decision + Layer 2 delivery to gateway.

Core principles reinforced:

• Layer 3 decides where traffic must go.
• Layer 2 decides how to reach the next hop.
• ARP resolves MAC addresses locally.
• Routers rewrite Layer 2 headers.
• IP addresses remain end-to-end.
• TTL decreases at each router.

This single topology clearly shows how host-to-host communication works at CCNA level without introducing VLAN complexity.

 
 

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