VLSM Address Plan Fault Investigation

Learning Objectives#

• Audit an enterprise Variable Length Subnet Masking (VLSM) address allocation plan against actual router interface and host workstation configurations.
• Identify invalid IPv4 host assignments, specifically detecting when directed broadcast or network identifier addresses are incorrectly assigned to endpoints.
• Diagnose and remediate default gateway and inter-router point-to-point transit subnet mask discrepancies using Cisco IOS diagnostic commands (show ip interface brief, show interfaces).
• Calculate exact prefix lengths, subnet masks, and usable host address ranges to satisfy host capacity requirements while preventing subnet overlap and conserving IPv4 address space.
• Verify bidirectional intra-subnet, inter-subnet, and cross-WAN transit data plane reachability across all provisioned organizational zones.

Prerequisites#

• Completion of PTL-0015: IPv4 Subnetting and VLSM Design.
• Understanding of binary and dotted-decimal subnet masks (/24, /26, /27, /28, /30).
• Familiarity with Cisco IOS router interface configuration and static route syntax.

Topology#

The topology models an enterprise branch network connected to corporate headquarters across an inter-router point-to-point transit link. Branch router R1 services two local departments (Engineering LAN on Gi0/0/0 and Sales LAN on Gi0/0/1) through dedicated access switches (SW-Eng and SW-Sales). Router R1 connects to HQ router R2 across a point-to-point WAN link on Gi0/0/2, while R2 provides access to the corporate Server DMZ on Gi0/0/0 through SW-DMZ.

+-----------------------------------------------------------------------------------+
|                                  ENTERPRISE TOPOLOGY                              |
|                                                                                   |
|  [Engineering LAN: /26]          [Sales LAN: /27]             [Server DMZ: /28]   |
|   PC-Eng1: .10                    PC-Sales1: .70               Srv-Web: .98       |
|        |                               |                            |             |
|     SW-Eng                          SW-Sales                      SW-DMZ          |
|        |                               |                            |             |
|      Gi0/0/0                         Gi0/0/1                      Gi0/0/0         |
|   +----------+                   +----------+                 +----------+        |
|   |    R1    |=== Gi0/0/2 ======>|    R2    |=================|  Servers |        |
|   | (Branch) |   [WAN: .112/30]  |   (HQ)   |                 +----------+        |
|   +----------+                   +----------+                                     |
+-----------------------------------------------------------------------------------+

Addressing Table#

Base Allocation: 192.168.100.0/24#

Device Interface Configuration Table#

Lab Tasks#

1. Audit the VLSM Address Allocation Plan: Review the enterprise base network 192.168.100.0/24 and verify that each departmental subnet block provides sufficient host capacity without overlapping adjacent subnets.
2. Investigate Endpoint IP Configuration Faults: Inspect PC-Eng1 network settings using ipconfig and identify why attempts to reach the local default gateway fail. Determine if the assigned address violates host addressing rules (network ID or broadcast address).
3. Audit Router Interface Subnet Masks: Run show ip interface brief and show interfaces on branch router R1. Compare the configured subnet masks on Gi0/0/1 and Gi0/0/2 against the formal VLSM address plan.
4. Remediate Addressing Discrepancies:
   • Reconfigure PC-Eng1 with a valid usable host IP (192.168.100.10/26).
   • Correct the subnet mask on R1 Gi0/0/1 to /27 (255.255.255.224).
   • Correct the subnet mask on R1 Gi0/0/2 to /30 (255.255.255.252).
5. Verify Intra-Subnet and Inter-Router Connectivity: Verify ping reachability from PC endpoints to their respective default gateways and across the point-to-point WAN transit link between R1 and R2.
6. Validate End-to-End Multi-Subnet Data Plane Delivery: Perform end-to-end ping tests from PC-Eng1 and PC-Sales1 to corporate server Srv-Web (192.168.100.98) to confirm full routing convergence.

Configuration Walkthrough#

1. Diagnosing Fault 1: Directed Broadcast Host Assignment#

In the starter topology, test gateway connectivity from PC-Eng1:

PC> ping 192.168.100.1
Pinging 192.168.100.1 with 32 bytes of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.100.1:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss)

Now inspect the IP configuration on PC-Eng1:

PC> ipconfig
FastEthernet0 Connection:(default port)
   Link-local IPv6 Address.........: FE80::201:97FF:FEAA:1101
   IP Address......................: 192.168.100.63
   Subnet Mask.....................: 255.255.255.192
   Default Gateway.................: 192.168.100.1

Analyze the subnet boundary for 192.168.100.0/26:

• A /26 prefix uses 26 network bits and 6 host bits ($2^6 = 64$ addresses per block).
• Block range: 192.168.100.0 to 192.168.100.63.
• Network address: 192.168.100.0.
• Usable host range: 192.168.100.1 through 192.168.100.62.
• Directed broadcast address: 192.168.100.63.

Because PC-Eng1 was assigned .63, it was configured with the directed broadcast address of the subnet. IP stacks reject unicast communication originating from or addressed to a broadcast address.

Remediation: Reconfigure PC-Eng1 with usable IP 192.168.100.10, mask 255.255.255.192, and gateway 192.168.100.1.

2. Diagnosing Fault 2: Subnet Capacity Deficit and Mask Truncation on Gateway#

Inspect router R1 interface settings:

R1# show interfaces GigabitEthernet0/0/1
GigabitEthernet0/0/1 is up, line protocol is up
  Hardware is ISR4331-3x1GE, address is 0002.166d.c402 (bia 0002.166d.c402)
  Description: Sales LAN
  Internet address is 192.168.100.65/28

Notice that R1 Gi0/0/1 is configured with 192.168.100.65/28 (255.255.255.240). However:

• The Sales LAN requires 25 host addresses. A /28 prefix provides only $2^4 - 2 = 14$ usable host addresses (192.168.100.65 through 192.168.100.78), with .79 acting as the directed broadcast address. This creates an immediate capacity deficit.
• In the starter topology, PC-Sales1 was assigned 192.168.100.85/27. Under the /28 mask configured on R1, address .85 is considered outside the local subnet. Consequently, PC-Sales1 cannot communicate with gateway 192.168.100.65 (100% packet loss).
• Correcting R1 Gi0/0/1 to /27 (255.255.255.224) expands the subnet block to 192.168.100.64 - 192.168.100.95 (30 usable hosts), providing sufficient capacity and restoring gateway reachability.

Remediation:

R1# configure terminal
R1(config)# interface GigabitEthernet0/0/1
R1(config-if)# ip address 192.168.100.65 255.255.255.224
R1(config-if)# exit

And reconfigure PC-Sales1 to planned address 192.168.100.70, mask 255.255.255.224, default gateway 192.168.100.65.

3. Diagnosing Fault 3: WAN Transit Prefix Inefficiency#

Inspect the inter-router point-to-point link on R1:

R1# show interfaces GigabitEthernet0/0/2
GigabitEthernet0/0/2 is up, line protocol is up
  Description: WAN Transit to R2
  Internet address is 192.168.100.113/29

R1 Gi0/0/2 was configured with /29 (255.255.255.248), whereas R2 Gi0/0/2 was correctly configured with /30 (255.255.255.252). A point-to-point transit link interconnecting only two routers requires exactly 2 usable host addresses ($2^2 - 2 = 2$), which is perfectly satisfied by a /30 prefix. Using /29 wastes 4 valuable host addresses.

Remediation:

R1(config)# interface GigabitEthernet0/0/2
R1(config-if)# ip address 192.168.100.113 255.255.255.252
R1(config-if)# exit
R1(config)# end
R1# copy running-config startup-config

Verification#

1. Interface IP and Subnet Mask Verification#

Verify the running interface table on R1:

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0/0   192.168.100.1   YES manual up                    up      
GigabitEthernet0/0/1   192.168.100.65  YES manual up                    up      
GigabitEthernet0/0/2   192.168.100.113 YES manual up                    up      

Confirm exact prefix lengths on R1:

• Gi0/0/0: Internet address is 192.168.100.1/26
• Gi0/0/1: Internet address is 192.168.100.65/27
• Gi0/0/2: Internet address is 192.168.100.113/30

Verify R2:

• Gi0/0/0: Internet address is 192.168.100.97/28
• Gi0/0/2: Internet address is 192.168.100.114/30

2. Intra-Subnet Gateway Ping Verification#

Test gateway reachability from PC-Eng1:

PC> ping 192.168.100.1
Pinging 192.168.100.1 with 32 bytes of data:
Reply from 192.168.100.1: bytes=32 time<1ms TTL=255
Reply from 192.168.100.1: bytes=32 time<1ms TTL=255
Reply from 192.168.100.1: bytes=32 time<1ms TTL=255
Reply from 192.168.100.1: bytes=32 time<1ms TTL=255

Ping statistics for 192.168.100.1:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Test gateway reachability from PC-Sales1:

PC> ping 192.168.100.65
Pinging 192.168.100.65 with 32 bytes of data:
Reply from 192.168.100.65: bytes=32 time<1ms TTL=255
Reply from 192.168.100.65: bytes=32 time<1ms TTL=255
Reply from 192.168.100.65: bytes=32 time<1ms TTL=255
Reply from 192.168.100.65: bytes=32 time<1ms TTL=255

Ping statistics for 192.168.100.65:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

3. Point-to-Point WAN Transit Verification#

Test connectivity across the WAN link from R1:

R1# ping 192.168.100.114
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.100.114, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/2 ms

4. End-to-End Enterprise Multi-Subnet Routing#

Verify multi-hop reachability from PC-Eng1 to corporate server Srv-Web:

PC> ping 192.168.100.98
Pinging 192.168.100.98 with 32 bytes of data:
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126

Ping statistics for 192.168.100.98:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Verify multi-hop reachability from PC-Sales1 to corporate server Srv-Web:

PC> ping 192.168.100.98
Pinging 192.168.100.98 with 32 bytes of data:
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126
Reply from 192.168.100.98: bytes=32 time=1ms TTL=126

Ping statistics for 192.168.100.98:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Verification Criteria Checklist#

Troubleshooting#

• PC Cannot Ping Local Gateway (Request timed out):
  • Run ipconfig on the endpoint. Ensure the IP address is not the network address (first address in block) or directed broadcast address (last address in block).
  • Verify that the subnet mask entered on the PC matches the prefix length configured on the router gateway interface.
  • Verify physical cabling and port status on the intermediate switch.
• Router Rejects Interface IP Configuration (% Overlaps with...):
  • Cisco IOS dynamically detects overlapping IP address blocks across active interfaces.
  • If a mask is too broad (e.g., using /24 or /26 when /27 was intended), the calculated subnet range will overlap adjacent interface blocks.
  • Correct the subnet mask or network block allocation before re-applying the IP address.
• Inter-Subnet Pings Fail Across WAN Transit Link:
  • Verify that both routers have static or dynamic routes for remote departmental subnets.
  • Check the point-to-point transit link: verify that both router interfaces share the same /30 subnet (255.255.255.252) and can ping each other directly.

Key Takeaways#

• VLSM Efficiency: VLSM maximizes IPv4 address efficiency by breaking away from classful and fixed-length boundaries, allocating subnets customized to specific departmental requirements ($2^h - 2 \ge \text{required hosts}$).
• Address Boundary Rules: In every IPv4 subnet, the first address ($h=0$) is reserved as the Network ID and the last address ($h=\text{all 1s}$) is reserved as the Directed Broadcast. Neither can ever be assigned to a host or router interface.
• Subnet Mask Consistency: Both the endpoint and the default gateway must share the identical subnet mask. A mask mismatch leads to split-horizon broadcast calculation errors and broken ARP resolution.
• Point-to-Point Optimization: Dedicated dual-node point-to-point links (such as router-to-router WAN circuits) should always use /30 (or /31 RFC 3021 where supported) to eliminate address waste.

References#

• RFC 1878: Variable Length Subnet Table For IPv4
• Cisco Technical Support: IP Addressing and Subnetting for New Users
• Cisco 4000 Series ISR Software Configuration Guide

Files and Validation Status#

• assets/starter.pkt: Initial topology containing addressing design faults (broadcast IP host assignment on Engineering PC, /28 mask truncation on Sales gateway with isolated host, and /29 mask on WAN link).
• assets/completed.pkt: Fully remediated topology adhering to the enterprise VLSM address plan.
• assets/topology.png: Visual network diagram illustrating branch and HQ router subnets.
• assets/test_evidence.json: Automated test execution results captured via Packet Tracer MCP bridge.
• configs/R1.txt: Baseline running configuration for Branch Router 1.
• configs/R2.txt: Baseline running configuration for HQ Router 2.
• verification.md: Comprehensive verification matrix detailing positive, negative, and reload tests.