BIG Subnet Design Exercise 1: Plan, Choose, and Configure IP Subnets and Addresses
IP Subnetting includes a variety of math processes that answer specific questions. But how do those processes then apply to building networks? In this BIG exercise, I’ll pull some of those ideas together. You’ll use the general processes to find all subnets of a network and then find the range of addresses in a specific subset of those subnets. You’ll also choose specific IP addresses to use in each subnet and create configurations in a text editor. As a final bonus, if you know how to configure IP addresses on routers, you can configure a lab to implement your plan using Cisco Packet Tracer. Enjoy!
The Lab Exercise
Background
This exercise uses a network as shown in Figure 1. It has several LANs, each with only the default VLAN 1 used on each LAN switch, meaning only one subnet is needed for each LAN in the figure. It also shows three WAN links. As a result, the design calls for a total of eight subnets, as shown in Figure 2.
Figure 1: Exercise Topology
Figure 2: Locations that Need Subnets for This Exercise
Tasks 1 & 2: Subnet Planning
Your first two tasks require you to use two subnetting processes:
- Find All Subnets of a Network, Assuming One Given Mask
- Find the Range of Usable Addresses in a Subnet
For task 1, apply that first process to the following table of three classful networks. For each network, the table lists the one mask used to create all subnets of that network. Use the process that reveals all the subnet IDs of the network – but you do NOT need to list all subnet IDs. For this exercise, list the first three (numerically-lowest three) and last three (numerically-highest three) subnets.
| Network | Subnet Mask |
|---|---|
| 10.0.0.0 | /20 |
| 192.168.9.0 | /30 |
| 172.20.0.0 | /23 |
Table 1: Networks and Masks. List All Subnets Created by the Masks.
Using the lists you just made, for task 2, choose specific subnets. Use the info in Table 2 to guide your choices. Basically, I want you to choose the same subnet IDs I choose for each location. So, for network 10.0.0.0, I ask you to use the lowest two subnets. In Network 192.168.9.0, I ask you to use the highest three subnets. For network 172.20.0.0, it’s back to the lowest subnets.
| Generic | Network | Mask | Which Subnet? |
|---|---|---|---|
| Subnet 11 | 10.0.0.0 | /20 | Lowest |
| Subnet 12 | 10.0.0.0 | /20 | 2nd Lowest |
| Subnet 21 | 192.168.9.0 | /30 | Highest |
| Subnet 22 | 192.168.9.0 | /30 | 2nd Highest |
| Subnet 23 | 192.168.9.0 | /30 | 3rd Highest |
| Subnet 31 | 172.20.0.0 | /23 | Lowest |
| Subnet 32 | 172.20.0.0 | /23 | 2nd Lowest |
| Subnet 33 | 172.20.0.0 | /23 | 3rd Lowest |
Table 1: Generic Subnets. Task: Pick Specific Subnets for Each Location.
Tasks 3 & 4: Address Planning
For Task 3, choose specific IP addresses to use for each PC, switch, and router in each LAN subnet. For task 4, do the same job, just for the WAN subnets.
For Task 3, use the following approach for the addresses to find and use for the LAN-based subnets:
- Find the four key facts about each subnet, which include the range of usable IP addresses
- Use the lowest address for the PC.
- Use the highest address for the router.
- Use the 2nd-highest address for the switch. (All switches use the VLAN 1 interface for their IP configuration.)
- Consider and record the IP addresses the PC and switch should use as their default gateway.
Record that information so it’s available for upcoming steps.
For Task 4, do similar tasks, but for the WAN subnets between the routers. Specifically:
- Find the four key facts about each subnet, which include the range of usable IP addresses
- Use the higher IP address for router R10.
- Use the lower IP address for the other router on the WAN link (R1, R2, or R3).
Again, record that information so it’s available for upcoming steps.
Tasks 5 & 6: Configure the Devices
I almost didn’t create this exercise because of the configuration tasks versus the order of the course topics. If you just learned about subnetting in my books or my matching course, you have not yet learned about configuring IP addresses on routers. However, you should understand the idea of routers using an IP address and mask setting on each interface. So I’ve made a compromise for this exercise:
- Step 5: Attempt to create the correct router and switch config for the IP addresses and default gateways. If you feel uncertain because you have not yet learned the required skills, take notes on the addresses and other values to use. Later in your studies, come back and try again. For the PCs, take notes about the addresses, masks, and default gateway settings.
- Step 6: Configure all the features using Cisco Packet Tracer. Clearly, step 6 makes sense if you already have the related IOS configuration skills. (Do not feel bad if you do not have those skills yet! This can be a great lab after just a few more chapters in the book, for instance.)
If you decide to try step 6, use the packet tracer file that you can download via the following link.
Exercise Intro Video
The above lab intro – the text, figures, and initial configuration – tells you all you need to know. But if you want a little more, with a little different slant on what to do in this lab, watch this exercise intro video!
Lab Answers Below: Spoiler Alert
Lab Answers: Configuration (Click Tab to Reveal)
Tasks 1 and 2
For Task 1, you should have listed the first and last several subnets of three different networks. The next three tables list the first three and last three subnets of those networks, respectively. The tables also list the range of usable addresses in each subnet, useful for tasks 3 and 4.
| Description | Subnet ID | First Address | Last Address | Broadcast Address |
|---|---|---|---|---|
| Lowest | 10.0.0.0 | 10.0.0.1 | 10.0.15.254 | 10.0.15.255 |
| 2nd Lowest | 10.0.16.0 | 10.0.16.1 | 10.0.31.254 | 10.0.31.255 |
| 3rd Lowest | 10.0.32.0 | 10.0.32.1 | 10.0.47.254 | 10.0.47.255 |
| 3rd Highest | 10.255.208.0 | 10.255.208.1 | 10.255.223.254 | 10.255.223.255 |
| 2nd Highest | 10.255.224.0 | 10.255.224.1 | 10.255.239.254 | 10.255.239.255 |
| Highest | 10.255.240.0 | 10.255.240.1 | 10.255.255.254 | 10.255.255.255 |
Table 3: Subnets of Network 10.0.0.0 with Mask /23
| Description | Subnet ID | First Address | Last Address | Broadcast Address |
|---|---|---|---|---|
| Lowest | 172.20.0.0 | 172.20.0.1 | 172.20.1.254 | 172.20.1.255 |
| 2nd Lowest | 172.20.2.0 | 172.20.2.1 | 172.20.3.254 | 172.20.3.255 |
| 3rd Lowest | 172.20.4.0 | 172.20.4.1 | 172.20.5.254 | 172.20.5.255 |
| 3rd Highest | 172.20.250.0 | 172.20.250.1 | 172.20.251.254 | 172.20.251.255 |
| 2nd Highest | 172.20.252.0 | 172.20.252.1 | 172.20.253.254 | 172.20.253.255 |
| Highest | 172.20.254.0 | 172.20.254.1 | 172.20.255.254 | 172.20.255.255 |
Table 4: Subnets of Network 172.20.0.0 with Mask /25
| Description | Subnet ID | First Address | Last Address | Broadcast Address |
|---|---|---|---|---|
| Lowest | 192.168.9.0 | 192.168.9.1 | 192.168.9.2 | 192.168.9.3 |
| 2nd Lowest | 192.168.9.4 | 192.168.9.5 | 192.168.9.6 | 192.168.9.7 |
| 3rd Lowest | 192.168.9.8 | 192.168.9.9 | 192.168.9.10 | 192.168.9.11 |
| 3rd Highest | 192.168.9.244 | 192.168.9.245 | 192.168.9.246 | 192.168.9.247 |
| 2nd Highest | 192.168.9.248 | 192.168.9.249 | 192.168.9.250 | 192.168.9.251 |
| Highest | 192.168.9.252 | 192.168.9.253 | 192.168.9.254 | 192.168.9.255 |
Table 5: Subnets of Network 192.168.9.0 with Mask /30
In Task 2, you needed to pick subnets from the lists you made (similar to my tables 3, 4, and 5 above), to match the earlier figure that listed the subnets generically as subnets 11, 12, 21, and so on. The next table shows the choices.
| Generic | Network | Mask | Which Subnet? | Chosen Subnet |
|---|---|---|---|---|
| Subnet 11 | 10.0.0.0 | /20 | Lowest | 10.0.0.0/20 |
| Subnet 12 | 10.0.0.0 | /20 | 2nd Lowest | 10.0.16.0/20 |
| Subnet 21 | 192.168.9.0 | /30 | Highest | 192.168.9.252/30 |
| Subnet 22 | 192.168.9.0 | /30 | 2nd Highest | 192.168.9.248/30 |
| Subnet 23 | 192.168.9.0 | /30 | 3rd Highest | 192.168.9.244/30 |
| Subnet 31 | 172.20.0.0 | /25 | Lowest | 172.20.0.0/23 |
| Subnet 32 | 172.20.0.0 | /25 | 2nd Lowest | 172.20.2.0/23 |
| Subnet 33 | 172.20.0.0 | /25 | 3rd Lowest | 172.20.4.0/23 |
Table 6: Subnets for Each Location
Tasks 3 & 4
For Tasks 3 and 4, you need to choose specific IP addresses from the far edges of the usable address ranges for each subnet. The tab for tasks 1 & 2 lists the ranges of usable addresses for many subnets. The next table below consolidates that data, listing only the eight subnets used in the current topology, listing the first and last addresses (the usable address range) for each subnet.
| Generic | Subnet ID | First Address | Last Address | Broadcast Address |
|---|---|---|---|---|
| Subnet 11 | 10.0.0.0 | 10.0.0.1 | 10.0.15.254 | 10.0.15.255 |
| Subnet 12 | 10.0.16.0 | 10.0.16.1 | 10.0.31.254 | 10.0.31.255 |
| Subnet 21 | 192.168.9.252 | 192.168.9.253 | 192.168.9.254 | 192.168.9.255 |
| Subnet 22 | 192.168.9.248 | 192.168.9.249 | 192.168.9.250 | 192.168.9.251 |
| Subnet 23 | 192.168.9.244 | 192.168.9.245 | 192.168.9.246 | 192.168.9.247 |
| Subnet 31 | 172.20.0.0 | 172.20.0.1 | 172.20.1.254 | 172.20.1.255 |
| Subnet 32 | 172.20.2.0 | 172.20.2.1 | 172.20.3.254 | 172.20.3.255 |
| Subnet 33 | 172.20.4.0 | 172.20.4.1 | 172.20.3.254 | 172.20.3.255 |
Table 7: Usable Address Range for the Subnets Used in this Exercise
Looking first to subnets 11 and 12, in network 10.0.0.0, on LANs off router R10, the exercise asks for the PC to use the lowest address, the router the highest address, and the switch the next highest address. The PC and switch use the router’s address as their default gateway. The next table summarizes those choices.
| Device | Address | Gateway |
|---|---|---|
| PC11 | 10.0.0.1 | 10.0.15.254 |
| R10 G0/1 | 10.0.15.254 | N/A |
| SW11 | 10.0.15.253 | 10.0.15.254 |
| PC12 | 10.0.16.1 | 10.0.31.254 |
| R10 G0/2 | 10.0.31.254 | N/A |
| SW12 | 10.0.31.253 | 10.0.31.254 |
Table 8: Address Plan for Subnets 11, 12
Looking next to LAN subnets 31, 32, and 33, on the right side of the topology, they follow the same convention: the PC uses the lowest address, the router the highest address, and the switch the next highest address. The next table summarizes those choices.
| Device | Address | Gateway |
|---|---|---|
| PC1 | 172.20.0.1 | 172.20.1.254 |
| R1 G0/0 | 172.20.1.254 | N/A |
| SW1 | 172.20.1.253 | 172.20.1.254 |
| PC2 | 172.20.2.1 | 172.20.3.254 |
| R2 G0/0 | 172.20.3.254 | N/A |
| SW2 | 172.20.3.253 | 172.20.3.254 |
| PC3 | 172.20.4.1 | 172.20.5.254 |
| R3 G0/0 | 172.20.5.254 | N/A |
| SW3 | 172.20.5.253 | 172.20.5.254 |
Table 9: Address Plan for Subnets 31, 32, 33
Moving on to WAN subnets 21, 22, and 23, with a /30 mask, those subnets support only two IP addresses per subnet. Each WAN link connects router R10 to one of the other routers. The instructions tell us to give router R10 the higher IP address in each case. The next table lists the choices.
| Generic Subnet | R10 | Other Router |
|---|---|---|
| Subnet 21 | 192.168.9.254 | 192.168.9.253 (R1) |
| Subnet 22 | 192.168.9.250 | 192.168.9.249 (R2) |
| Subnet 23 | 192.168.9.246 | 192.168.9.245 (R3) |
Table 10: Address Plan for WAN Subnets 21, 22, 23
Tasks 5 & 6
This exercise poses a problem for those of you who are attempting this exercise along with my books and course: You haven’t learned all the related device configuration steps yet. So, step 5 gives you a chance to collect as much configuration data as you can, without worrying about whether it is entirely correct and could be used in a device. Step 6 offers a higher challenge level to anyone who knows how to configure IP settings on routers and switches and PC IP settings in Packet Tracer.
For the purposes of reviewing Tasks 5 and 6, I decided to show the switch and router configuration as IOS commands with correct syntax. Even if you have never seen the router configuration, the syntax is clear just looking at the examples. The rest of this section offers configurations for those switches and routers.
interface GigabitEthernet0/0
ip address 172.20.1.254 255.255.254.0
!
interface GigabitEthernet0/0/0
ip address 192.168.9.253 255.255.255.252Example 1: R1 Config
interface GigabitEthernet0/0
ip address 172.20.3.254 255.255.254.0
!
interface GigabitEthernet0/0/0
ip address 192.168.9.249 255.255.255.252Example 2: R2 Config
interface GigabitEthernet0/0
ip address 172.20.5.254 255.255.254.0
!
interface GigabitEthernet0/0/0
ip address 192.168.9.245 255.255.255.252
Example 3: R3 Config
interface GigabitEthernet0/1
ip address 10.0.15.254 255.255.240.0
!
interface GigabitEthernet0/2
ip address 10.0.31.254 255.255.240.0
!
interface GigabitEthernet0/1/0
ip address 192.168.9.254 255.255.255.252
!
interface GigabitEthernet0/2/0
ip address 192.168.9.250 255.255.255.252
!
interface GigabitEthernet0/3/0
ip address 192.168.9.246 255.255.255.252Example 4: R10 Config
interface vlan1
ip address 10.0.15.253 255.255.240.0
!
ip default-gateway 10.0.15.254Example 5: SW11 Config
interface vlan1
ip address 10.0.31.253 255.255.240.0
!
ip default-gateway 10.0.31.254Example 6: SW12 Config
interface vlan1
ip address 172.20.1.253 255.255.254.0
!
ip default-gateway 172.20.1.254Example 7: SW1 Config
interface vlan1
ip address 172.20.3.253 255.255.254.0
!
ip default-gateway 172.20.3.254Example 8: SW2 Config
interface vlan1
ip address 172.20.5.253 255.255.254.0
!
ip default-gateway 172.20.5.254Example 9: SW3 Config
Exercise Review Video
Want to hear more about this exercise’s solution? Check out the video to the left.