Config Lab: IPv4 Addresses 5
More practice practice practice! This time, you get some basic IP addressing requirements. Your job: calculate the IP addresses to be used by routers and hosts, and create the router configuration for in the interfaces in the network diagram. No guile, no tricks, just a chance to exercise.
The Lab Exercise
Requirements
This lab asks you to configure all the router IP addresses for the design shown in Figure 1, given a set of requirements. At this point, do not configure static routes or a routing protocol; just configure the IP addresses per the following specific instructions:
- Use the subnets as listed in Figure 1.
- Routers must use the highest IP address(es) in each subnet.
- If more than one router exists in a subnet, the router with the higher number in the hostname gets the higher IPv4 address. EG, R3 would have a higher IPv4 address than R2.
- Hosts should use the lowest IPv4 address in the subnet, with the same tiebreaker concept as stated in #2.
Figure 1: Router Triangle with IPv4 Subnets
Table 1 shows a way to organize the addresses for reference.
| Location | Address/Mask |
| R1 G0/0 | |
| R1 G0/2/0 | |
| R1 G0/3/0 | |
| R2 G0/0 | |
| R2 G0/1/0 | |
| R2 G0/3/0 | |
| R3 G0/0 | |
| R3 G0/1/0 | |
| R3 G0/2/0 |
Table 1: IPv4 Addresses
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:
- Download the .pkt file linked below.
- Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
- Add your planned configuration to the lab.
- Test the configuration using some of the suggestions below.
This Lab Supports Both CML-Free and CML-Personal!!!
The downloadable file listed here works in both CML-P or CML-F because it uses the IOL (router) and IOL-L2 (switch) reference platform images supported by both products as of CML V2.8. Note that these images also require less CPU and RAM than the other CML-P options.
Use the same general workflow as with Cisco Packet Tracer, as follows:
- Download the CML file (filetype .yaml) linked below.
- Import the lab’s CML file into CML.
- Start the lab in CML.
- Compare the CML lab topology and interface IDs to this lab Blog page, as they may differ (more detail below).
- Add your planned configuration to the lab, adjusting for interface ID differences.
- Test the configuration using some of the suggestions below.
Download this lab’s CML file!
Interface ID Differences:
Any lab diagrams shown with this lab use interface IDs per the conventions of the Packet Tracer version of the lab. When using CML, adjust the lab interfaces used based on this table. Also, note that the IOL and IOL-L2 images use interface type “Ethernet”, not “FastEthernet” or “GigabitEthernet”.
| Device Type |
Lab Port | Â CML Port |
| Router | G0/0 | E0/0 |
| Router | G0/1/0 | E0/1 |
| Router | G0/2/0 | E0/2 |
| Router | G0/3/0 | E0/3 |
Lab Answers Below: Spoiler Alert
Lab Answers: Configuration (Click Tab to Reveal)
Answers
Your first task was to calculate the IP addresses to use, based on requirements. For reference, Table 1 lists the results. Note that all subnets use a /26 (255.255.255.192) mask in this particular exercise.
| Location | IP/mask |
| R1 G0/0 | 172.18.1.62 /26 |
| R1 G0/2/0 | 172.18.8.125 /26 |
| R1 G0/3/0 | 172.18.8.189 /26 |
| R2 G0/0 | 172.18.2.126 /26 |
| R2 G0/1/0 | 172.18.8.126 /26 |
| R2 G0/3/0 | 172.18.8.253 /26 |
| R3 G0/0 | 172.18.3.190 /26 |
| R3 G0/1/0 | 172.18.8.190 /26 |
| R3 G0/2/0 | 172.18.8.254 /26 |
Table 1: IPv4 Addresses
To create the configuration, all you have to do is add the ip address command to each interface in configuration mode. Easy once you’d done it a few times; here are the answers to be complete.
interface GigabitEthernet0/0
ip address 172.18.1.62 255.255.255.192
!
interface GigabitEthernet0/2/0
ip address 172.18.8.125 255.255.255.192
!
interface GigabitEthernet0/3/0
ip address 172.18.8.189 255.255.255.192Example 1: R1 IPv4 Address Configuration
interface GigabitEthernet0/0
ip address 172.18.2.126 255.255.255.192
!
interface GigabitEthernet0/1/0
ip address 172.18.8.126 255.255.255.192
!
interface GigabitEthernet0/3/0
ip address 172.18.8.253 255.255.255.192Example 2: R2 IPv4 Address Configuration
interface GigabitEthernet0/0
ip address 172.18.3.190 255.255.255.192
!
interface GigabitEthernet0/1/0
ip address 172.18.8.190 255.255.255.192
!
interface GigabitEthernet0/2/0
ip address 172.18.8.254 255.255.255.192Example 3: R3 IPv4 Address Configuration
Commentary, Issues, and Verification Tips (Click Tabs to Reveal)
This lab focuses on the math to determine the numerically highest IP addresses in a subnet. So, check your math versus the table in the answers section.
The lab does request that you configure the routers to match the plan. By doing so, and while also NOT using a routing protocol to learn routes, you can do some testing of your choices of IP addresses. However, to do a basic check, examine the ending configuration with the show running-config command to ensure the addresses and masks you intended to configure are in the configuration. Note that the masks should all be 255.255.255.192.
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.
Known Issues in this Lab w/ CML
This tab lists known issues with running this lab in CML with the supplied file. The issues are:
| # | Summary | Detail |
| 1 | Lab file uses a “Router-as-Many-Hosts” approach | The lab diagram shows multiple PCs. However, the CML file implements all hosts as one virtualized router. From that one router, you can issue ping and traceroute commands as if they were done on a host. Check out this video for more information about this approach. |
Why Would CML Have Issues?
(Note: The text below is the same as every Config Lab.)
CML supports a variety of Cisco operating systems (called reference platforms.) To make them work in CML, Cisco makes some adjustments to the code. Also, because no real router or switch hardware exists, some software features do not work the same when running in CML versus a real Cisco device. When we come across any difference when testing the lab, we’ll try to leave a note just above in case it helps you with the 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.Â
- Ping the IP address on the other end of each WAN link. To do so, from each router, try these steps:
- Login to that router and ping that router’s local IP addresses on its three interfaces – the ones you just configured. All the ping should work.
- Examine the figure, your notes about your planned IP addresses, and even the configuration on the other routers, to remind yourself of the IP addresses on the other end of the WAN links connected to the local router. EG, from R1, find R2’s IP address on its G0/1/0 port, and R3’s IP address on its G0/1/0 port.
- From the local router, ping the IP address on the other end of each WAN link. If you configured the address on the local and remote ends of the WAN link to be in the same subnet, the ping should work.
- Examine Connected Routes. From each router:
- Issue the show ip route command and focus on the connected routes only.
- Compare the command output to the figure for this lab. The figure lists the subnets and masks, and the command output should as well. Compare and ensure that the command output lists the same subnet IDs and masks shown in the figure.
Hello Wendell,
Please a question. I am going through this question from PTP and do not quite understand the explanation for the host route with two routes. Why will the route to server 2 not default to the floating route after the route with the better AD fails? I thought the concept of floating route is to take over if the route with the better AD fails? I have attached a screenshot of the question and posted the answer below:
This question has many details to work through, but before thinking about the answers, focus on the five ip route commands in the exhibit. Look for the two commands that list 172.16.12.2 as the next-hop router address. Per the figure and question stem, R1’s G0/1 interface fails, which is the interface that leads to neighbor address 172.16.12.2.
Continuing to look at the two ip route commands with 172.16.12.2 as next-hop address, you can ignore one for this question because R1 will remove it from the routing table when R1’s G0/1 interface fails. However, the route in the ip route command with the permanent keyword remains in the IP routing table (because of the permanent keyword), even though the interface the route uses is down.
Next, move on to think about how a router uses the administrative distance (AD) of routes. IOS uses the AD as a tiebreaker when two competing routes have the exact same destination and prefix. In this case, two host routes list the exact same destination (172.16.20.2, which is server S2) and exact same mask (255.255.255.255, meaning both are host routes). One route does not list an AD, so it uses the default AD for static IP routes of 1. The other lists an AD of 140. So, on this decision point, IOS prefers the AD 1 route. Unfortunately, this one route (with the ip route 172.16.20.2 172.16.12.2 permanent command) remains in the routing table even though the next-hop address is not reachable.
As a result of this analysis, packets sent to server S2 (172.16.20.2) match the host route that uses 172.16.12.2 as the next hop address. But because that address is not currently reachable, R1 discards the packet.
As for packets sent to server S1 (172.16.10.1), only one of the five routes match the destinations as configured in the five ip route commands in the exhibit. One route configures a destination that comes close to matching address 172.16.10.1: the ip route 172.16.8.0 255.255.254.0 172.16.13.3 command. This command matches addresses 172.16.8.0–172.16.9.255. (Note that the figure lists server S1’s subnet as 172.16.8.0/21, so a correct static route for the entire subnet should have used dotted decimal mask 255.255.248.0.) So packets sent to server S1’s 172.16.10.1 address will match the default route and be sent to router R4 (172.16.14.4) next.
Hi Tewa,
It’s a challenging question. First, a request. There’s a feedback button in the PTP user interface. Those flow to me, along with a link to the question. That’s a little easier way from this end to answer queries about PTP questions (for future reference.)
Anyway, two static routes rely on the G0/1 interface fails. However, one such route has the permanent keyword. So, a failure of interface G0/1, which means there’s no route to reach neighbor address 172.16.12.2, does NOT remove the route from R1;s routing table. So, when a packet arrives w/ server S2’s address as destination, R1 matches the route that lists 172.16.12.2 as the next-hop address. Then the router has no ability to forward the packet to that next hop address, so it drops the packet.
Hope this helps,
Wendell
I am either misunderstanding Rule #4 or I got the answer right ?
Rule #4 states that “Hosts should use the lowest IPv4 address in the subnet, with the same tiebreaker concept as stated in #2.”
Wouldn’t R1’s G 0/0 interface be 172.18.1.1 255.255.255.192 instead of 172.18.1.62/26 since 1 is lower than 62?
R2’s G0/0 interface should be 172.18.2.65 /26 instead of 172.18.2.126 /26
R3’s G0/0 interface should be 172.18.3.129 /26 instead of 172.18.3.190 /26
Please let me know if this is incorrect or not..?
I got the other interface IP addresses correct with the Router’s having the Highest IP address in the subnet w/ higher numbered Routers getting the higher IP address.
Hi Ryley,
Your math is perfect; it’s the interpretation of the wording that’s the issue.
#2 in the requirements states that routers use the numerically highest address in the subnet. Then #4 says hosts use the lowest addresses. So, routers use highest, hosts (aka not the routers, but the PCs in this case) use the lowest. Your notes suggest giving the routers the lowest addresses.
So, all good on the important part (the math). As for wording, literally, “host” means any device with an IP address, but commonly, “host” means “some endpoint device, like a PC, laptop, phone, etc)”, rather than a router or switch.
Hope this helps…
Wendell,
Yes that makes sense, would R1-3 interfaces still use the lower number for the router’s interface towards the hosts or would that just apply to the host’s IP address themselves.
I comprehend that the interfaces are still on the router yet since it’s the path for the host to connect through the router, would the IP address on G0/0 R1 (for example) be the lowest address..?
(I choose to believe the host is a “tablet” in this lab haha!)
V/R
Ryley Trepagnier
Hello again,
So, let’s take R1’s G0/0. It is the only router interface connected to that VLAN. Per #2 in the list of requirements, it gets the highest IP address in the subnet. So the lab description isn’t based on the end-to-end path, just where the address is assigned.
Hope this helps,
Wendell
Hi Mr. Odom,
I understand since G0/0 is on R1, I guess i am trying to ask if that would effect end-to-end path, more or less. If it does, would the IP address on R1’s G0/0 interface need to reflect the host’s IP address?
Thank you for answering my obnoxious questions. These kinds of details help immensely sir .!
V/R
Ryley
Hi Ryley,
Guess I missed your note. So…
Short answer is no. There is no association between the host’s address and the router’s address, other than the fact that they need to be in the same subnet.
So, whatever’s got you thinking down this path… it’s not a thing! 🙂
Have a great weekend…
Wendell