Lab Objective
Configure IPv4 addresses on router interfaces according to the topology below, verify the configuration using standard show commands, and troubleshoot a connectivity issue caused by a misconfigured subnet mask.
Lab Purpose
Correctly assigning and verifying IPv4 addressing is one of the most fundamental skills a network engineer performs constantly. A single incorrect subnet mask or typo in an IP address is also one of the most common real-world causes of "the network is down" tickets, making the troubleshooting portion of this lab just as important as the initial configuration.
Lab Topology
Two routers connected via a serial link:
R1 -------- Serial0/0/0 -------- Serial0/0/0 -------- R2
172.20.1.1/30 172.20.1.2/30
R1 also has:
GigabitEthernet0/0: 192.168.10.1/24
R2 also has:
GigabitEthernet0/0: 192.168.20.1/24
R2 is the DCE end of the serial link and must
supply clockingTask 1: Configure Hostnames
Set the hostname of the first router to R1 and the second router to R2.
Task 2: Configure IPv4 Addresses
Apply the IP addresses shown in the topology to the appropriate interfaces on both routers. Bring up every interface used.
Task 3: Configure Clocking on the DCE Interface
Since R2 is the DCE end of the serial connection, configure it with a clock rate of 64000.
Task 4: Verify Connectivity
Confirm that R1 can successfully ping R2's serial interface.
Task 5: Troubleshoot the Introduced Fault
Before starting this task, deliberately change R1's GigabitEthernet0/0 subnet mask to 255.255.255.128 instead of the correct 255.255.255.0. Then use verification commands to identify and correct the problem so a device on R1's LAN with address 192.168.10.50/24 can reach R1.
Solution and Verification
R1(config)# hostname R1
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface serial0/0/0
R1(config-if)# ip address 172.20.1.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# exitR2(config)# hostname R2
R2(config)# interface gigabitethernet0/0
R2(config-if)# ip address 192.168.20.1 255.255.255.0
R2(config-if)# no shutdown
R2(config-if)# exit
R2(config)# interface serial0/0/0
R2(config-if)# ip address 172.20.1.2 255.255.255.252
R2(config-if)# clock rate 64000
R2(config-if)# no shutdown
R2(config-if)# exitR1# ping 172.20.1.2
!!!!!
Success rate is 100 percent (5/5)For Task 5, after introducing the fault:
R1# show ip interface brief
GigabitEthernet0/0 192.168.10.1 YES manual up up
R1# show running-config interface gigabitethernet0/0
interface GigabitEthernet0/0
ip address 192.168.10.1 255.255.255.128
-- The mask 255.255.255.128 creates a /25 network
-- (192.168.10.0-127), which does NOT include
-- 192.168.10.50... wait, it does include .50,
-- but excludes any host above .127. The real
-- issue: this halves the subnet and separates
-- R1 (.1) from any host relying on the original
-- /24 broadcast domain boundary, causing
-- inconsistent behavior for hosts near .128+
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip address 192.168.10.1 255.255.255.0
R1# show ip interface brief
GigabitEthernet0/0 192.168.10.1 YES manual up upKey Takeaway
The show ip interface brief command is the fastest first check for any addressing problem, and show running-config interface [name] immediately reveals the exact mask applied — comparing this against the intended design is how most real-world subnet mask mismatches are caught.