Config Lab: IPv4 Addresses 3

 In 200-301 V1 Ch16: Router Addresses and Routes, 200-301 V1 Part 5: IPv4 Routing, 200-301 V1 Parts, Config Lab, Config Lab CCNA Vol 1 Part 4, Config Lab CCNA Vol 1 Part 5, Hands-on

With enough repetition, you should be able to imagine a small network of routers, choose a subnetting plan, choose addresses for the router interfaces, and configure those addresses, all pretty much as fast as you can write or type. This latest config lab exercise helps you practice towards that goal. It lists a subnetting design and asks you to find the lowest IP addresses in each of six subnets and configure the addresses on Cisco router interfaces. Details below the fold. Enjoy!

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


The following figure shows a simple internetwork with three routers. The figure also lists one IP address in six different subnets. These IP addresses are not the IP addresses used by the routers. Instead, these IP addresses are part of the subnets you should use for your configuration. First, you have to do some math.

Figure 1:Topology and Subnets for this Lab


Your jobs and rules:

  1. Calculate the numerically lowest IP addresses in each subnet based on the information in the figure.
  2. Configure the routers to:
    1. Bring up their interfaces.
    2. Use the lowest IP address in each subnet.
  3. For the subnets that include two routers, assign the lower-numbered router the lower IP address value.
  4. Each router should be able to successfully ping its three own IP addresses when complete.
  5. Use defaults unless absolutely necessary.


Initial Configuration

Assume that the routers have no configuration to begin the lab other than their hostnames.

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:

This table lists the interfaces used in the lab exercise documentation that differ from those used in the sample CML file.

Device Lab Port  CML Port
R1 G0/2/0 G0/2
R1 G0/3/0 G0/3
R2 G0/1/0 G0/1
R2 G0/3/0 G0/3
R3 G0/1/0 G0/1
R3 G0/2/0 G0/2

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)


To find the lowest IP addresses, calculate the subnet ID, and then add 1 in the 4th octet. I won’t go into how to find the subnet ID – that’s all over the books – but I’ll entertain specific questions if you have them. Then you just need to configure the IP addresses using the ip address address mask interface subcommand, as shown here.

Figure 1: Three Router Topology w/ Sample IP Addresses for Calculations


interface GigabitEthernet0/0
 no shutdown
 ip address
interface GigabitEthernet0/2/0
 no shutdown
 ip address
interface GigabitEthernet0/3/0
 no shutdown
 ip address

Example 1: R1 Config


interface GigabitEthernet0/0
 no shutdown
 ip address
interface GigabitEthernet0/1/0
 no shutdown
 ip address
interface GigabitEthernet0/3/0
 no shutdown
 ip address

Example 2: R2 Config


interface GigabitEthernet0/0
 no shutdown
 ip address
interface GigabitEthernet0/1/0
 no shutdown
 ip address
interface GigabitEthernet0/2/0
 no shutdown
 ip address

Example 3: R3 Config

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

Before configuring any commands for this lab, you needed to plan your IP addresses. In this case, the figure shows the subnet IDs, and you have to find the numerically lowest address(es) to use for the routers. That makes the math pretty easy – just add one to the 4th octet of the subnet ID to get the numerically lowest IP address in the subnet.

Once planned, configuring the IP addresses require one command per interface: the ip address address mask command, with the address and dotted-decimal mask used as the two parameters.

Finally, for this lab, you should have considered adding the no shutdown command to each interface. The lab states that the routers have no configuration other than the hostname command. As a result, the interfaces may be in an administratively disabled (shutdown) state to begin the lab. The no shutdown command would then change the state to administratively enabled.

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 you should see only connected and local routes in the routing tables unless you chose to add routing protocol configuration. You can verify those routes as compared to the figure. On each router:
    1. Use the show ip route command. Look for three connected routes, one for each router interface per the figure.
  3. Use ping tests to ping the router IP addresses on the opposite end of each WAN link:
    1. From router R1, ping the R2 WAN IP address on their shared link and then the R3 WAN IP address on their shared link.
    2. Likewise, from router R2, ping R1’s WAN IP address and R3’s WAN IP address.
    3. Finally, from R3, ping R1’s and R2’s WAN IP addresses on the respective shared links.

More Labs with Related Content!

Config Lab: IPv4 Addresses 2
Config Lab: IPv4 Addresses 4
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I believe the config in the lab answers for R2 has the IP addresses for G0/1/0 and G0/3/0 swapped.

The R1 interface connected to R2’s G0/1/0 interface is, but the lab answer lists the IP address on R2 G0/1/0 as

Likewise, the address on R2’s G0/3/0 interface does not jibe with the address on the R3 side.


Yes indeed. Sorry about that, but thanks for the note. I’ve swapped the addresses.
Thanks again,

Frank Wijmans

Could be just me, but I cant import this lab into CML. It shows the error:

“description”: “Invalid Lab: Invalid schema version.”,
“code”: 400

Any idea on what’s going wrong?

Punya Athma

I’ve done all configrtns succesfully.
The, Ping, to each router’s own Fiber connector interfaces works. However until otherwise another device perhaps a PC, (or a Switch, Hub, or Router), connected to this Gig 0/0 interface of the Router, this Gig 0/0 line comes up but not the line protocol. Thereby I connected a PC to each router’s Gig 0/0 interface to make this interface work.

Awaiting comments earnestly.


So, if you began with the supplied .pkt (Packet Tracer) or .yaml (CML) file, that topology should show each router’s G0/0 connected to a LAN switch. So, the router G0/0 interfaces, respectively, should reach an up/up state. It seems like you’re saying in your testing you began with no devices at all, not even a LAN switch, connected to each router’s G0/0 interface?
If so, in that case, on real gear, a no shut would result in a down/down state. I re-created that case in Packet Tracer and see the up/down state, so to stress the fact, that’s NOT the same as I would expect on real gear. If the router has no cable connected, or a cable connected to a powered-off device, or nothing cabled to the other end, then a “no shut” interface should land in a down/down state. FYI.
But… connected to a powered-on switch, with it’s interface enabled, as is the case in the beginning state of this lab, the router G0/0 interfaces should reach an up/up state.
Feel free to clarify. Happy labbing…


This was great for practicing ip addressing for several subnets. Happy to say I got them all right per the lab answers. Although I am super slow and I needed time to figure out the SID’S and valid add’s. Gonna have to improve my speed for the exam and for real world projects.

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