IPv4 Subnetting and VLSM Network Design

In traditional classful or fixed-length subnet masking (FLSM), every subnet in an organization is forced to use the same subnet mask regardless of the actual number of connected hosts. This rigid design causes severe IPv4 address waste—for example, dedicating a /24 (254 hosts) or /26 (62 hosts) subnet to a point-to-point router WAN link that only requires two IP addresses.

Variable Length Subnet Masking (VLSM) eliminates this inefficiency by allowing network engineers to recursively divide a network block into subnets of varying prefix lengths (/26, /27, /28, /30).

In this lab, you will design, calculate, configure, and verify an end-to-end multi-site network derived from a single classful 192.168.1.0/24 network block using VLSM principles.

Learning Objectives#

• Understand the business necessity and technical mechanics of Variable Length Subnet Masking (VLSM).
• Partition a base /24 network block to satisfy specific host requirements using /26, /27, /28, and /30 prefix lengths.
• Calculate network addresses, valid host address ranges, broadcast addresses, and decimal subnet masks.
• Configure Cisco router interfaces and end-host operating systems with custom VLSM masks.
• Verify end-to-end ICMP reachability across variably subnetted broadcast domains and point-to-point transit links.
• Analyze Cisco IOS validation mechanisms for overlapping subnets and reserved host addresses.

Prerequisites#

• Completion of PTL-0002: Configuring Cisco Router Interfaces and PTL-0008: Cisco IOS Navigation and Initial Device Configuration.
• Mastery of binary-to-decimal conversion and powers of 2.
• Understanding of the difference between network, usable host, and broadcast IP addresses.

Topology#

The enterprise network consists of two sites interconnected across a point-to-point WAN transit link:

• Headquarters Site (Router R1): Connects the large Staff LAN via access switch S1 (192.168.1.2) and workstation PC-Staff (192.168.1.10).
• Branch Office Site (Router R2): Connects the small Branch LAN via access switch S2 (192.168.1.114) and workstation PC-Branch (192.168.1.120).
• WAN Transit Link: A dedicated point-to-point interconnection connecting router interface R1 Gi0/0/1 (192.168.1.97) to R2 Gi0/0/1 (192.168.1.98).

Addressing Table#

VLSM Subnet Allocation Scheme (Base Network: 192.168.1.0/24)#

Device Interface Assignment Table#

Lab Tasks#

1. Calculate VLSM Subnets: Using the base 192.168.1.0/24 block, determine the prefix lengths and ranges from largest host requirement to smallest.
2. Examine Starter State: Open assets/starter.pkt and verify that the WAN point-to-point link (192.168.1.96/30) and static inter-router routes are pre-configured.
3. Configure HQ Staff Gateway: Assign 192.168.1.1 255.255.255.192 to R1 GigabitEthernet0/0/0 and activate the interface.
4. Configure Branch Gateway: Assign 192.168.1.113 255.255.255.240 to R2 GigabitEthernet0/0/0 and activate the interface.
5. Configure Switch SVIs: Assign management IP addresses and default gateways to S1 and S2.
6. Configure Host Workstations: Assign IP parameters to PC-Staff (192.168.1.10/26) and PC-Branch (192.168.1.120/28).
7. Perform End-to-End Testing: Ping across the routed WAN boundary from PC-Staff to PC-Branch.
8. Investigate Subnet Constraints: Observe Cisco IOS error messages when attempting to assign a broadcast address or overlapping subnet.

Configuration Walkthrough#

1. Router R1 Configuration#

Configure the Staff LAN gateway interface (/26) on router R1:

R1> enable
R1# configure terminal
R1(config)# interface GigabitEthernet0/0/0
R1(config-if)# description Staff LAN Gateway (/26)
R1(config-if)# ip address 192.168.1.1 255.255.255.192
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# end
R1# write memory

2. Router R2 Configuration#

Configure the Branch LAN gateway interface (/28) on router R2:

R2> enable
R2# configure terminal
R2(config)# interface GigabitEthernet0/0/0
R2(config-if)# description Branch LAN Gateway (/28)
R2(config-if)# ip address 192.168.1.113 255.255.255.240
R2(config-if)# no shutdown
R2(config-if)# exit
R2(config)# end
R2# write memory

3. Switch S1 and S2 Management Setup#

Configure S1:

S1> enable
S1# configure terminal
S1(config)# interface Vlan1
S1(config-if)# ip address 192.168.1.2 255.255.255.192
S1(config-if)# no shutdown
S1(config-if)# exit
S1(config)# ip default-gateway 192.168.1.1
S1(config)# end
S1# write memory

Configure S2:

S2> enable
S2# configure terminal
S2(config)# interface Vlan1
S2(config-if)# ip address 192.168.1.114 255.255.255.240
S2(config-if)# no shutdown
S2(config-if)# exit
S2(config)# ip default-gateway 192.168.1.113
S2(config)# end
S2# write memory

4. Workstation Configuration#

In the Packet Tracer Desktop tab:

• PC-Staff: IP 192.168.1.10, Subnet Mask 255.255.255.192, Default Gateway 192.168.1.1.
• PC-Branch: IP 192.168.1.120, Subnet Mask 255.255.255.240, Default Gateway 192.168.1.113.

Verification#

Positive Test 1: PC-Staff IP Configuration (/26)#

C:\>ipconfig

FastEthernet0 Connection:
   IPv4 Address....................: 192.168.1.10
   Subnet Mask.....................: 255.255.255.192
   Default Gateway.................: 192.168.1.1

Positive Test 2: PC-Branch IP Configuration (/28)#

C:\>ipconfig

FastEthernet0 Connection:
   IPv4 Address....................: 192.168.1.120
   Subnet Mask.....................: 255.255.255.240
   Default Gateway.................: 192.168.1.113

Positive Test 3: Local Gateway Reachability#

From PC-Staff, ping R1:

C:\>ping 192.168.1.1
Reply from 192.168.1.1: bytes=32 time<1ms TTL=255

Positive Test 4: WAN Transit Point-to-Point Ping#

From R1, ping R2:

R1# ping 192.168.1.98
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.98, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 0/0/0 ms

Positive Test 5: End-to-End Inter-Subnet Host Ping#

From PC-Staff, ping PC-Branch:

C:\>ping 192.168.1.120

Pinging 192.168.1.120 with 32 bytes of data:

Reply from 192.168.1.120: bytes=32 time<1ms TTL=126
Reply from 192.168.1.120: bytes=32 time<1ms TTL=126
Reply from 192.168.1.120: bytes=32 time<1ms TTL=126
Reply from 192.168.1.120: bytes=32 time<1ms TTL=126

Ping statistics for 192.168.1.120:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss)

Note: TTL=126 confirms that the packet successfully traversed two router hops (R1 and R2).

Positive Test 6: Inspect VLSM Routing Table Output#

R1# show ip route
Gateway of last resort is not set

     192.168.1.0/24 is variably subnetted, 5 subnets, 4 masks
C       192.168.1.0/26 is directly connected, GigabitEthernet0/0/0
L       192.168.1.1/32 is directly connected, GigabitEthernet0/0/0
C       192.168.1.96/30 is directly connected, GigabitEthernet0/0/1
L       192.168.1.97/32 is directly connected, GigabitEthernet0/0/1
S       192.168.1.112/28 [1/0] via 192.168.1.98

Notice: The routing table explicitly states variably subnetted, 5 subnets, 4 masks. This is the signature hallmark of a live VLSM network!

Troubleshooting#

Key Takeaways#

• Subnet From Largest to Smallest: When designing a VLSM address plan, always allocate subnets to the largest host requirements first, descending to the smallest point-to-point links.
• Save Address Space with /30: Using /30 (255.255.255.252) for point-to-point router links uses only 4 total addresses (2 usable), preventing the waste of dozens of IP addresses.
• Custom Masks: Unlike classful networks (/8, /16, /24), VLSM subnet masks split octets at non-zero values (.128, .192, .224, .240, .248, .252).
• Overlapping Subnet Detection: Cisco IOS actively protects against misconfiguration by preventing overlapping subnet assignments on active routed interfaces.

References#

• RFC 1878: Variable Length Subnet Table For IPv4
• Cisco IP Addressing and Subnetting for New Users
• Repository Troubleshooting Guide

Files and Validation Status#