Topology
Objectives
Part 1: Examine Network Requirements Part 2: Design the VLSM Address Scheme Part 3: Cable and Configure the IPv4 Network
Background / Scenario
Variable Length Subnet Mask (VLSM) was designed to avoid wasting IP addresses. With VLSM, a network is subnetted and then re-subnetted. This process can be repeated multiple times to create subnets of various sizes based on the number of hosts required in each subnet. Effective use of VLSM requires address planning. In this lab, use the 172.16.128.0/17 network address to develop an address scheme for the network displayed in the topology diagram. VLSM is used to meet the IPv4 addressing requirements. After you have designed the VLSM address scheme, you will configure the interfaces on the routers with the appropriate IP address information. Note: The routers used with CCNA hands-on labs are Cisco 1941 Integrated Services Routers (ISRs) with Cisco IOS Release 15.2(4)M3 (universalk9 image). Other routers and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the commands available and output produced might vary from what is shown in the labs. Refer to the Router Interface Summary Table at the end of this lab for the correct interface identifiers. Note: Make sure that the routers have been erased and have no startup configurations. If you are uncertain, contact your instructor.
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| 8.2.1.5 Lab – Designing And Implementing A VLSM Addressing Scheme |
Required Resources
3 routers (Cisco 1941 with Cisco IOS software, Release 15.2(4)M3 universal image or comparable) 1 PC (with terminal emulation program, such as Tera Term, to configure routers) Console cable to configure the Cisco IOS devices via the console ports Ethernet (optional) and serial cables, as shown in the topology
Windows Calculator (optional)
Part 1: Examine Network Requirements
In Part 1, you will examine the network requirements to develop a VLSM address scheme for the network displayed in the topology diagram using the 172.16.128.0/17 network address. Note: You can use the Windows Calculator application and the http://www.ipcalc.org IP subnet calculator to help with your calculations.
Step 1: Determine how many host addresses and subnets are available.
How many host addresses are available in a /17 network? 32766__ What is the total number of host addresses needed in the topology diagram? 31506 host____ How many subnets are needed in the network topology? 9
Step 2: Determine the largest subnet.
What is the subnet description (e.g. BR1 G0/1 LAN or BR1-HQ WAN link)? HQ-G0/0_________ How many IP addresses are required in the largest subnet? 16000____ What subnet mask can support that many host addresses? __m = (2^n) – 2 >= 16000 -> m= 14 MSn = 32-14 /18_______________ How many total host addresses can that subnet mask support? (2^n) – 2 >= 16000 = 16382_______ Can you subnet the 172.16.128.0/17 network address to support this subnet? yes___ What are the two network addresses that would result from this subnetting? 172.16.128.0/18_________________ 172.16.192.0/18_________________ Use the first network address for this subnet.
Step 3: Determine the second largest subnet.
What is the subnet description? ___HQ-G0/1______________________ How many IP addresses are required for the second largest subnet? 8000__ What subnet mask can support that many host addresses? m = (2^n) – 2 >= 8000 = m = 13 MSn = 32-13 /19___________ How many total host addresses can that subnet mask support? 8190____ Can you subnet the remaining subnet again and still support this subnet? yes__ What are the two network addresses that would result from this subnetting? 172.16.192.0 /19 ___ 172.16.1224.0 /19 ___ Use the first network address for this subnet.
Step 8: Determine the subnets needed to support the serial links.
How many host addresses are required for each serial subnet link? 2 What subnet mask can support that many host addresses? /30_________ a. Continue subnetting the first subnet of each new subnet until you have four /30 subnets. Write the first three network addresses of these /30 subnets below. 172.16.254.0/30____ 172.16.254.4/30____ 172.16.254.8/30____ b. Enter the subnet descriptions for these three subnets below. __ HQ S0/0/0 – BR1 S0/0/0______ __ HQ S0/0/1 – BR2 S0/0/1______ __ BR1 S0/0/1 – BR2 S0/0/0______
Part 2: Design the VLSM Address Scheme
Step 1: Calculate the subnet information.
Use the information that you obtained in Part 1 to fill in the following table
Subnet Description
Number of Hosts Needed
Network Address /CIDR
First Host Address
Broadcast Address
HQ G0/0 16,000 172.16.128.0/
172.16.128.1/
8 172.16.191.255/
HQ G0/1 8,000 172.16.192.0 /
172.16.192.1/
9 172.16.223.255/
BR1 G0/1 4,000 172.16.224.0/
172.16.224.1/
0 172.16.239.255/
BR1 G0/0 2,000 172.16.240.0/
172.16.240.1/
1 172.16.247.255/
BR2 G0/1 1,000 172.16.248.0/
172.16.248.1/
2 172.16.251.255/
BR2 G0/0 500 172.16.252.0/
172.16.252.1/
3 172.16.253.255/
HQ S0/0/0 – BR1 S0/0/0 2 172.16.254.0/
172.16.254.1/
0 172.16.254.3/
HQ S0/0/1 – BR2 S0/0/1 2 172.16.254.4/
172.16.254.5/
0 172.16.254.7/
BR1 S0/0/1 – BR2 S0/0/0 2 172.16.254.8/
172.16.254.9/
0 172.16.254.11/
Step 2: Complete the device interface address table.
Assign the first host address in the subnet to the Ethernet interfaces. HQ should be given the first host address on the Serial links to BR1 and BR2. BR1 should be given the first host address for the serial link to BR2.
Step 4: Save the configuration on all devices.
Step 5: Test Connectivity.
a. From HQ, ping BR1’s S0/0/0 interface address. b. From HQ, ping BR2’s S0/0/1 interface address. c. From BR1, ping BR2’s S0/0/0 interface address. d. Troubleshoot connectivity issues if pings were not successful. Note: Pings to the GigabitEthernet interfaces on other routers will not be successful. The LANs defined for the GigabitEthernet interfaces are simulated. Because no devices are attached to these LANs they will be in down/down state. 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. These interfaces will remain in a down/down state until a device is connected to the other end of the Ethernet interface cable. The focus of this lab is on VLSM and configuring the interfaces.
Reflection
Can you think of a shortcut for calculating the network addresses of consecutive /30 subnets? The magic number, that consists of raising 2 to the power that corresponds to the number of bits, of this form we can obtain the next subnet address __
Router Interface Summary Table
Router Interface Summary
Router Model Ethernet Interface #1 Ethernet Interface #2 Serial Interface #1 Serial Interface # 1800 Fast Ethernet 0/ (F0/0)
Fast Ethernet 0/ (F0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
1900 Gigabit Ethernet 0/ (G0/0)
Gigabit Ethernet 0/ (G0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2801 Fast Ethernet 0/ (F0/0)
Fast Ethernet 0/ (F0/1)
Serial 0/1/0 (S0/1/0) Serial 0/1/1 (S0/1/1)
2811 Fast Ethernet 0/ (F0/0)
Fast Ethernet 0/ (F0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2900 Gigabit Ethernet 0/ (G0/0)
Gigabit Ethernet 0/ (G0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
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.
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