11.10.2 Lab – Design and Implement a VLSM Addressing Scheme (Answers)

Objectives Part 1: Analyze Network Requirements 

Part 2: Construct the VLSM Address Scheme

Part 3: Configure and Cable the IPv4 Network

Context / Scenario

Variable Length Subnet Mask (VLSM) was created to prevent IP address wastage. A network is subnetted and subsequently re-subnetted using VLSM. This procedure may be run numerous times to generate subnets of varying sizes depending on the desired number of hosts per subnet. Effective usage of VLSM involves planning of addresses.

Use the network address 192.168.33.128.0/25 to construct an address scheme for the network seen in the topology diagram. VLSM is used to satisfy the IPv4 addressing specifications. After designing the VLSM address scheme, the interfaces on the routers will be configured with the proper IP address information. Addresses will need to be assigned to the future LANS at BR2, but no interfaces will be setup at this time.

Note: Cisco 4221 routers running Cisco IOS XE Release 16.9.4 are used in CCNA hands-on labs (universalk9 image). The lab switches are Cisco Catalyst 2960s running Cisco IOS Release 15.2(2). (lanbasek9 image). Other routers, switches, and versions of Cisco IOS may be used. Depending on the model and Cisco IOS version, the available commands and output may differ from what is shown in the labs. For the right interface IDs, see the Router Interface Summary Table at the conclusion of the lab.

Note: Ensure that the routers have been cleared of any starting settings and have been wiped. If you are uncertain, contact your teacher.

Refer to the Instructor Lab Manual for instructions on how to initialise and reload devices.

If time is a concern, this lab may be done in numerous sessions. Parts 1 and 2 are administered on paper and may be given as homework. Part 3 is Hands-on and involves lab equipment.

Essential Resources

Two Routers (Cisco 4221 with Cisco IOS XE Release 16.9.4 universal image or comparable)

2 Switches (Cisco 2960 equipped with Cisco IOS Release 15.2(2) lanbasek9 image or equivalent)

◾1 PCs (Windows with terminal emulation program, such as Tera Term)

Console cables used to configure Cisco IOS devices using their console ports

As demonstrated in the topology, Ethernet and serial cables

Microsoft Windows Calculator (optional)

Instructions

1. Analyze Network Requirements

In Part 1, you will investigate the network requirements for developing a VLSM address scheme for the network shown in the topology diagram with the network address 192.168.33.128.0/25.

You may use the Windows Calculator programme or the Internet to locate an IP subnet calculator to assist you with your calculations.

Determine the number of available host addresses and subnets.

How many accessible host addresses does a /25 network have?

126

How many host identifiers are required for the topology diagram?

80

How many subnets are required for the network architecture?

6

Determine the biggest subnet in Step 2.

What is the subnet description (BR1 LAN or BR2-BR2 connection, for example)?

BR1 LAN

How many IP addresses must the biggest subnet have?

40

Which subnet mask can accommodate so many host addresses?

/26 or 255.255.255.192

How many host addresses can this subnet mask accommodate in total?

62

Can the network address 192.168.33.128.0/25 be subnetted to support this subnet?

yes

What network addresses would be generated by this subnetting?

192.168.33.128/26 and 192.168.33.192/26

Utilize the initial network address for the subnet.

Determine the second-largest subnet in Step 3.

What is the description of the subnet?

BR2 LAN

How many IP addresses must the second-largest subnet have?

25

Which subnet mask can accommodate so many host addresses?

/27 or 255.255.255.224

How many host addresses can this subnet mask accommodate in total?

30

Can the remaining subnet be subnetted again while still supporting this subnet?

yes

What network addresses would be generated by this subnetting?

192.168.33.192/27 and 192.168.33.224/27

Utilize the initial network address for the subnet.

Determine the third biggest subnet in Step 4.

What is the description of the subnet?

IoT BR2 LAN

How many IP addresses will the next-largest subnet require?

5

Which subnet mask can accommodate so many host addresses?

/29 or 255.255.255.248

How many host addresses can this subnet mask accommodate in total?

6

Can the remaining subnet be subnetted again while still supporting this subnet?

yes

What network addresses would be generated by this subnetting?

192.168.33.224/29, 192.168.33.232/29, 192.168.33.240/29, and 192.168.33.248/29

Utilize the initial network address for the subnet.

Utilize the second network address for the CCTV local area network.

Utilize the third IP address for the HVAC C2 LAN.

Determine the fourth biggest subnet in step 5.

What is the description of the subnet?

BR1-BR2 Link

How many IP addresses will the next-largest subnet require?

2

Which subnet mask can accommodate so many host addresses?

/30 or 255.255.255.252

How many host addresses can this subnet mask accommodate in total?

2

Can the remaining subnet be subnetted again while still supporting this subnet?

yes

What network addresses would be generated by this subnetting?

192.168.33.248/30 and 192.168.33.252/30

Utilize the initial network address for the subnet.

Developing the VLSM Address Scheme

Calculate the subnet information in the first step.

Utilize the information gained in Part 1 to complete the table below.

Step 2: Complete the device interface address table.

Assign the first host address in the subnet to the Ethernet interfaces. BR1 should be assigned the first host address in the BR1-BR2 Link.

Part 3: Cable and Configure the IPv4 Network

In Part 3, you will cable the network to match the topology and configure the three routers using the VLSM address scheme that you developed in Part 2.

Step 1: Cable the network as shown in the topology.

Step 2: Configure basic settings on each router.

a. Assign the device name to the routers.
router(config)# hostname BR1
router(config)# hostname BR2

b. Disable DNS lookup to prevent the routers from attempting to translate incorrectly entered commands as though they were hostnames.
BR1(config)# no ip domain lookup
BR2(config)# no ip domain lookup

c. Assign class as the privileged EXEC encrypted password for both routers.
BR1(config)# enable secret class
BR2(config)# enable secret class

d. Assign cisco as the console password and enable login for the routers.
BR1(config)

line con 0

BR1(config-line)# password cisco
BR1(config)# login
BR2(config)

line con 0

BR2(config-line)# password cisco
BR2(config)# login

e. Assign cisco as the VTY password and enable login for the routers.

BR1(config)# line vty 0 4
BR1(config-line)# password cisco
BR1(config-line)# login
BR2(config)# line vty 0 4
BR2(config-line)

password cisco

BR2(config-line)

login

f. Encrypt the plaintext passwords for the routers.
BR1(config)# service password-encryption
BR2(config)# service password-encryption

g. Create a banner that will warn anyone accessing the device that unauthorized access is prohibited on both routers.
BR1(config)# banner motd $ Unauthorized Access is Prohibited $
BR2(config)# banner motd $ Unauthorized Access is Prohibited $

Step 3: Configure the interfaces on each router.

a. Assign an IP address and subnet mask to each interface using the table that you completed in Part 2.
BR1(config)# interface g0/0/0
BR1(config-if)# ip address 192.168.33.249 255.255.255.252
BR1(config-if)# interface g0/0/1
BR1(config-if)# ip address 192.168.33.129 255.255.255.192
BR2(config)# interface g0/0/0
BR2(config-if)# ip address 192.168.33.250 255.255.255.252
BR2(config-if)# interface g0/0/1
BR2(config-if)# ip address 192.168.33.192 255.255.255.224

b. Configure an interface description for each interface.
BR1(config)# interface g0/0/0
BR1(config-if)

description BR1-BR2 Link

BR1(config-if)# interface g0/0/1
BR1(config-if)# description Connected to S1
BR2(config-if)

interface g0/0/0

BR2(config-if)# description BR1-BR2 Link
BR2(config-if)

interface g0/0/1

BR2(config-if)# description Connected to S2

c. Activate the interfaces.

BR1(config)

interface g0/0/0

BR1(config-if)# no shutdown
BR1(config-if)

interface g0/0/1

BR1(config-if)# no shutdown
BR2(config)

interface g0/0/0

BR2(config-if)# no shutdown
BR2(config-if)

interface g0/0/1

BR2(config-if)# no shutdown

Step 4: Save the configuration on all devices.
BR1# copy running-config startup-config
BR2# copy running-config startup-config

Step 5: Test Connectivity.

a. From BR1, ping BR2’s G0/0/0 interface.

b. From BR2, ping BR1’s G0/0/0 interface.

c. Troubleshoot connectivity issues if pings were not successful.

Note: Pings to the GigabitEthernet LAN interfaces on other routers will not be successful. A routing protocol needs to be in place for other devices to be aware of those subnets. The GigabitEthernet interfaces also need to be in an up/up state before a routing protocol can add the subnets to the routing table. The focus of this lab is on VLSM and configuring the interfaces.

Reflection Question

Can you think of a shortcut for calculating the network addresses of consecutive /30 subnets?

Answers may vary. A /30 network has 4 address spaces: the network address, 2 host addresses, and a broadcast address. Another technique for obtaining the next /30 network address would be to take the network address of the previous /30 network and add 4 to the last octet.

Router Interface Summary Table

Note: To find out how the router is configured, look at the interfaces to identify the type of router and how many interfaces the router has. There is no way to effectively list all the combinations of configurations for each router class. This table includes identifiers for the possible combinations of Ethernet and Serial interfaces in the device. The table does not include any other type of interface, even though a specific router may contain one. An example of this might be an ISDN BRI interface. The string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands to represent the interface.

Device Configs

Router BR1 (Final Configuration)

BR1# show run
Building configuration...


Current configuration : 1558 bytes
!
version 16.9
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
no platform punt-keepalive disable-kernel-core
!
hostname BR1
!
boot-start-marker
boot-end-marker
!
!
vrf definition Mgmt-intf
 !
 address-family ipv4
 exit-address-family
 !
 address-family ipv6
 exit-address-family
!
enable secret 5 $1$ehVu$efQjBqXqcVcSfwLJyhwHT/
!
no aaa new-model
!
no ip domain lookup
!
subscriber templating
!
multilink bundle-name authenticated
!
spanning-tree extend system-id
!
redundancy
 mode none
!
interface GigabitEthernet0/0/0
 description BR1-BR2 Link
 ip address 192.168.33.249 255.255.255.252
 negotiation auto
!
interface GigabitEthernet0/0/1
 description Connected to S1
 ip address 192.168.33.129 255.255.255.192
 negotiation auto
!
interface Serial0/1/0
!
interface Serial0/1/1
!
interface GigabitEthernet0
 vrf forwarding Mgmt-intf
 no ip address
 shutdown
 negotiation auto
!
ip forward-protocol nd
no ip http server
no ip http secure-server
ip tftp source-interface GigabitEthernet0
!
control-plane
!
banner motd ^C Unauthorized Access is Prohibited ^C
!
line con 0
 password 7 121A0C041104
 login
 stopbits 1
line aux 0
 stopbits 1
line vty 0 4
 password 7 045802150C2E
 login
!
end

Router BR2 (Final Configuration)

BR2# show run
Building configuration...

Current configuration : 1468 bytes
!
version 16.9
service timestamps debug datetime msec
service timestamps log datetime msec
service password-encryption
no platform punt-keepalive disable-kernel-core
!
hostname BR2
!
boot-start-marker
boot-end-marker
!
vrf definition Mgmt-intf
 !
 address-family ipv4
 exit-address-family
 !
 address-family ipv6
 exit-address-family
!
enable secret 5 $1$.s3c$IJxdfZCYkvll8ifXtsW8O/
!
no aaa new-model
!
no ip domain lookup
!
subscriber templating
!
multilink bundle-name authenticated
!
spanning-tree extend system-id
!
redundancy
 mode none
!
interface GigabitEthernet0/0/0
 description BR1-BR2 Link
 ip address 192.168.33.250 255.255.255.252
 negotiation auto
!
interface GigabitEthernet0/0/1
 description Connected to S2
 ip address 192.168.33.193 255.255.255.224
 negotiation auto
!
interface Serial0/1/0
!
interface Serial0/1/1
!
interface GigabitEthernet0
 vrf forwarding Mgmt-intf
 no ip address
 shutdown
 negotiation auto
!
ip forward-protocol nd
no ip http server
no ip http secure-server
ip tftp source-interface GigabitEthernet0
!
control-plane
!
banner motd ^C Unauthorized Access is Prohibited ^C
!
line con 0
 password 7 0822455D0A16
 login
 stopbits 1
line aux 0
 stopbits 1
line vty 0 4
 password 7 070C285F4D06
 login
!
end