Subnetting Verification and Ping Test
Lab Objectives
- Verify IPv4 addressing and SVIs using show ip interface brief across routers and switches.
- Test end-to-end connectivity with ping and analyze path behavior with traceroute.
- Interpret results to confirm correct subnetting and identify common misconfiguration causes.
ASCII Topology (BASE LAB TOPOLOGY — exact IPs shown)
[Internet]
203.0.113.1
|
R1 (Gateway)
Gi0/0: 10.10.10.1
Gi0/1: 10.10.20.1
Gi0/2: 10.10.30.1
/ | \
R2 R3 R4
Gi0/0: 10.10.10.2 | Gi0/0: 10.10.30.2
Gi0/1: 10.10.40.1 |
/ \ |
S1 S2 S3
/ \ | /
PC1 PC2 PC3 PC4 PC5
IP addressing summary:
- Router links: 10.10.10.0/24, 10.10.20.0/24, 10.10.30.0/24, 10.10.40.0/24
- VLANs (subnets): 192.168.1.0/24 (VLAN 10 Sales), 192.168.2.0/24 (VLAN 20 Engineering), 192.168.3.0/24 (VLAN 30 Management)
- Internet: 203.0.113.0/24
Tip: Think of a SVI (Switch Virtual Interface) as the switch’s IP on a VLAN — it is the default gateway for hosts in that VLAN (like an apartment building mailroom for each floor).
Lab Tasks (Try It Yourself First!)
Complete these tasks WITHOUT looking at the solution below. Use
?andshowcommands to figure it out.
Task 1: Verify device addressing and interface state
On each router (R1–R4) and the access switches hosting SVIs (S1–S3), confirm the configured IPv4 addresses and that the interfaces are up. Note mismatches between expected subnets and actual addresses.
Task 2: Basic reachability tests (Ping)
From R2:
- Ping R1 (10.10.10.1) and R4 (10.10.30.2). From a host in VLAN 10 (PC1 with 192.168.1.11):
- Ping PC3 (192.168.2.11).
- Ping Internet address 203.0.113.1.
Task 3: Path analysis (Traceroute)
From R2: run traceroute to 203.0.113.1 and interpret each hop. From a management host (PC5 in VLAN 30): traceroute to 192.168.1.11 and verify routing through the gateway(s).
Think About It: Why does a traceroute show each router hop even when a ping succeeds? What additional diagnostic information does traceroute reveal that ping does not?
Lab Solution
Task 1 Solution: Verify device addressing and interface state
What we are doing: We will confirm the IP addresses and operational state of interfaces on routers and switches using the show ip interface brief command. This quickly identifies IP mismatches or down interfaces.
! On each router / switch
show ip interface brief
What just happened:
show ip interface brieflists interfaces, assigned IP addresses, and status (up/down).- This reveals whether the interface is administratively up (no shutdown) and whether the line protocol is operational.
Verify:
R1# show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.10.10.1 YES manual up up
GigabitEthernet0/1 10.10.20.1 YES manual up up
GigabitEthernet0/2 10.10.30.1 YES manual up up
Loopback0 203.0.113.254 YES manual up up
R2# show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.10.10.2 YES manual up up
GigabitEthernet0/1 10.10.40.1 YES manual up up
R3# show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.10.20.2 YES manual up up
R4# show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.10.30.2 YES manual up up
S1# show ip interface brief
Interface IP-Address OK? Method Status Protocol
Vlan10 192.168.1.1 YES manual up up
S2# show ip interface brief
Vlan20 192.168.2.1 YES manual up up
S3# show ip interface brief
Vlan30 192.168.3.1 YES manual up up
Why this matters: If an interface is down or IP is wrong, hosts in that subnet cannot communicate beyond the local link. This step isolates addressing errors early.
Task 2 Solution: Basic reachability tests (Ping)
What we are doing: Use ping to verify connectivity between routers and from hosts to remote subnets and the internet. Ping tests confirm basic L3 reachability and subnet correctness.
! From R2 to R1 and R4
R2# ping 10.10.10.1
R2# ping 10.10.30.2
! From R2 to Internet
R2# ping 203.0.113.1
! From a host (PC1) - simulated from switch using extended ping with source
S1# ping 192.168.2.11 source 192.168.1.1
S1# ping 203.0.113.1 source 192.168.1.1
What just happened:
ping <addr>sends ICMP Echo Requests; successive replies indicate connectivity.- Using
sourcesimulates a ping from the VLAN gateway when you cannot run commands on a PC.
Verify:
R2# ping 10.10.10.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/2/4 ms
R2# ping 10.10.30.2
!!!!!
Success rate is 100 percent (5/5)
S1# ping 192.168.2.11 source 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.2.11, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
S1# ping 203.0.113.1 source 192.168.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 203.0.113.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5)
Real-world context: In production, ping is the first tool for verifying whether routing and ACLs allow traffic between subnets and the internet. A failed ping directs you to check routing, default gateways, or ACLs.
Task 3 Solution: Path analysis (Traceroute)
What we are doing: Use traceroute to show each hop between source and destination, identifying where packets are forwarded or dropped.
R2# traceroute 203.0.113.1
S3# traceroute 192.168.1.11
What just happened:
traceroutesends packets with increasing TTL; each router that decrements TTL to zero returns an ICMP Time Exceeded message, revealing the path.- This helps locate where traffic leaves the local network and which router is the egress.
Verify:
R2# traceroute 203.0.113.1
Type escape sequence to abort.
Tracing the route to 203.0.113.1
1 10.10.10.1 1 msec 1 msec 1 msec
2 203.0.113.1 2 msec 2 msec 2 msec
S3# traceroute 192.168.1.11
Type escape sequence to abort.
Tracing the route to 192.168.1.11
1 10.10.30.1 1 msec 1 msec 1 msec
2 10.10.10.1 2 msec 2 msec 2 msec
3 192.168.1.1 3 msec 3 msec 3 msec
Why traceroute matters: Unlike ping, traceroute shows intermediate hops. In production, this helps spot asymmetric routing or a specific router dropping traffic.
Troubleshooting Scenario
Scenario: Wrong subnet mask on VLAN gateway
Symptom: Ping from PC1 (192.168.1.11) to PC3 (192.168.2.11) fails; traceroute shows traffic reaching the local gateway but not beyond.
Your task: Find and fix the issue.
Hint: Check the SVI mask for VLAN 10.
Solution:
- On S1, the SVI for VLAN10 was configured with a /16 mask by mistake. Fix to /24:
S1# configure terminal
S1(config)# interface Vlan10
S1(config-if)# ip address 192.168.1.1 255.255.255.0
S1(config-if)# no shutdown
S1(config-if)# end
Explanation: A wrong mask places the host in a different network range, preventing correct routing to other VLAN subnets. Correcting the SVI mask restores proper subnet boundaries.
Verification Checklist
- show ip interface brief confirms correct IPs and interfaces are up.
- ping between router-to-router and host-to-host succeed.
- traceroute shows the expected hop sequence through R1 to the Internet.
Common Mistakes
| Symptom | Cause | Fix |
|---|---|---|
| Ping to another VLAN times out | Host default gateway missing or incorrect SVI IP | Configure SVI with correct IP/mask and ensure default gateway on host points to it |
| traceroute stops at first hop | Static route or default route missing on gateway | Add default route to Internet on R1 (production: ensure correct next-hop) |
| Partial pings succeed intermittently | Duplex or physical interface flapping | Check interface status, negotiate / set duplex correctly, replace cable if needed |
Warning: In many labs, devices start with interfaces shutdown — always verify with
no shutdownon interfaces and SVIs.
Challenge Task
Without step-by-step guidance, configure R3 so that it provides a new subnet 192.168.4.0/24 for a new switch S4 and validate end-to-end connectivity from a host in 192.168.4.0/24 to the Internet (203.0.113.1). Ensure you use appropriate SVIs, static routing or routing protocols, and verify with ping and traceroute.