Config Lab: IPv4 Addresses 2

Wendell Odom
By Wendell Odom September 10, 2021 11:05

Repetition helps when learning IP addressing and subnetting. The next lab gives you more reps with subnetting math and with configuring router IP addresses. If you already know the math, make this one a speed test, and find out how long it takes you from the point of reading the specifics until you can type the config!

All about Config Labs

The blog has a series of lab exercises called “Config Labs.” Each lab presents a topology with the relevant initial configuration for each device. The lab also lists new requirements, after which you should create the additional configuration to meet those requirements. You can do the lab on paper, in a text editor, or use software tools like Cisco Packet Tracer or Cisco Modeling Labs.

Once you have created your answer, you can click various tabs at the bottom of this post to see the lab answers, comments about the lab, and other helpful information.

The Lab Exercise


Configure the IP addresses for the LAN interfaces for the routers shown in the figure per the subnets listed in the figure. The specific rules for this lab are:

  1. On each router’s G0/1 interface, use the last octet of 1, 2, 3, or 4 for routers R1, R2, R3, and R4, respectively.
  2. On each router’s G0/2 interface, configure each router with the highest allowable address in the subnet.
  • Assumptions:
    • Assume all device interfaces shown in the lab are up and working.
    • Assume that the PCs are pre-configured with the correct IP addresses.

Figure 1: Four Routers and Five Switches


Initial Configuration

Examples 1, 2, 3, and 4 show the beginning configuration state of R1, R2, R3, and R4.

Example 1: R1 Config


Example 2: R2 Config


Example 3: R3 Config


Example 4: R4 Config


Note also that the switch in the center of the network has all default configuration other than a hostname.

Answer Options - Click Tabs to Reveal

You can learn a lot and strengthen real learning of the topics by creating the configuration – even without a router or switch CLI. In fact, these labs were originally built to be used solely as a paper exercise!

To answer, just think about the lab. Refer to your primary learning material for CCNA, your notes, and create the configuration on paper or in a text editor. Then check your answer versus the answer post, which is linked at the bottom of the lab, just above the comments section.

You can also implement the lab using the Cisco Packet Tracer network simulator. With this option, you use Cisco’s free Packet Tracer simulator. You open a file that begins with the initial configuration already loaded. Then you implement your configuration and test to determine if it met the requirements of the lab.

(Use this link for more information about Cisco Packet Tracer.)

Use this workflow to do the labs in Cisco Packet Tracer:

  1. Download the .pkt file linked below.
  2. Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
  3. Add your planned configuration to the lab.
  4. Test the configuration using some of the suggestions below.

Download this lab’s Packet Tracer File

You can also implement the lab using Cisco Modeling Labs – Personal (CML-P). CML-P (or simply CML) replaced Cisco Virtual Internet Routing Lab (VIRL) software in 2020, in effect serving as VIRL Version 2.

If you prefer to use CML, use a similar workflow as you would use if using Cisco Packet Tracer, as follows:

  1. Download the CML file (filetype .yaml) linked below.
  2. Import the lab’s CML file into CML and then start the lab.
  3. Compare the lab topology and interface IDs to this lab, as they may differ (more detail below).
  4. Add your planned configuration to the lab.
  5. Test the configuration using some of the suggestions below.

Download this lab’s CML file!


Network Device Info:

The CML topology matches this lab topology exactly. The host info does as well.


Host device info:

This table lists host information pre-configured in CML/VIRL, information that the lab might not require but may be useful to you.


IP Address

Mac Address














Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)


Figure 1: Four Switches with Trunks


Example 5: R1 Config


Example 2: R2 Config


Example 3: R3 Config


Example 4: R4 Config

Commentary, Issues, and Verification Tips (Click Tabs to Reveal)


When configuring IP addressing information on a networking device, it is essential to ensure that the information is correct before putting a device into production. Unlike an IP overlap on a single PC which could affect that one device or possibly affect another PC on the local subnet, the misconfiguration of an IP address on a networking device can affect the whole LAN.

For this lab, you were tasked with performing the IP addressing configuration on the five subnets shown in the figure. Four of the subnets required that the router use the highest IP address in each subnet for the connecting interface, as follows:

  • R1’s LAN subnet uses the subnet. The range of addresses, including the subnet ID and broadcast address, is –, for a router interface address of
  • R2’s LAN subnet uses the subnet. The range of addresses, including the subnet ID and broadcast address, is –, for a router interface address of
  • R3’s LAN subnet uses the subnet. The range of addresses, including the subnet ID and broadcast address, is –, for a router interface address of
  • R4’s LAN subnet uses the subnet. The range of addresses, including the subnet ID and broadcast address, is –, for a router interface address of

Additionally, all four G0/1 interfaces needed an address in subnet

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

# Summary Detail
1 None No known issues related to this lab.


Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

  1. From the console of each router, verify the router’s IP addresses:
    1. Use the show ip interface command to verify the address and mask.
    2. Use the show ip interfaces brief command to verify the addresses.
    3. Use the ping command against the addresses expected to be configured on the local router; all pings should work.
  2. The initial configuration does not include any routing protocol configuration, so unless you chose to add routing protocol configuration, you should see only connected and local routes in the routing tables. You can verify those routes as compared to the figure. On each router:
    1. Use the show ip route command. Look for two connected routes – one connected to interface G0/1 and one connected to interface G0/2.
  3. Use ping tests to ping the router IP addresses in shared subnet From each router, ping all four router IP addresses in that subnet. All pings should work.

More Labs with Related Content!

Config Lab: IPv4 Addresses 1
Config Lab: IPv4 Addresses 3
Wendell Odom
By Wendell Odom September 10, 2021 11:05
Write a comment


  1. Emil February 3, 22:38

    Great lab for practicing subnetting! If you want all the PCs to be able to ping each other, run these 3 commands on each router to turn on OSFP (you’ll have to wait a minute before all routers advertise everything):

    conf t
    int range g0/1-2
    ip ospf 1 area 0
    no shut

    Reply to this comment
    • Emil February 4, 23:02

      Also noticed that there’s a duplicate of this lab in CONFIG LAB CCNA VOL 1 PART 5 > Config Lab: IPv4 Addresses 2.

      Reply to this comment
  2. boudjema October 1, 05:13

    hello and thank you for the lab
    please why i can’t ping from one Pc in one subnet to another Pc in another subnet ? event the 4 Routers have a static routes ??
    thank you

    Reply to this comment
    • certskills October 12, 17:27

      Well, without bringing up the lab in PT to test it, I’d say check the default gateway settings on the PCs. If unset, or set incorrectly, the PC-to-PC pings would fail.

      Reply to this comment
  3. Jay Mahannah January 29, 21:35

    The last octet on the Switch side for each router’s G0/1 interface should be .251-.254 instead of .1-.4, right?

    Reply to this comment
    • Wendell Odom Author February 3, 16:19

      I’m not following you. Can you clarify? The routers’ all have a G0/1 interface connected to a layer 2 switch. The layer 2 switch has no IP address that matters to the lab. The switch might have one IP address in that VLAN for management, but the lab is silent about suggesting a value. If you’re suggesting four different IP addresses for the switch, I’m confused. Clarify?

      Reply to this comment
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