Review Activities
Chapter Summary
Variable Length Subnet Masks (VLSM) simply means that the subnet design uses more than one mask in the same classful network.
VLSM provides many benefits for real networks, mainly related to how you allocate and use your IPv4 address space. Because a mask defines the size of the subnet (the number of host addresses in the subnet), VLSM enables engineers to better match the need for addresses with the size of the subnets.
VLSM can be helpful for both public and private IPv4 addresses, but the benefits are more dramatic with public networks. With public networks, the address savings help engineers avoid having to obtain another registered IPv4 network number from regional IP address assignment authorities.
In addition, more advanced classless routing protocols use VLSM to support manual route summarization, which enables a routing protocol to advertise one route for a larger subnet instead of multiple routes for smaller subnets.
To support VLSM, the routing protocol must advertise the mask along with each subnet. Without mask information, the router receiving the update would be unable to determine network address from host address.
By definition, classless routing protocols advertise the mask with each advertised route, and classful routing protocols do not.
Cisco routers do not configure VLSM, enable or disable it, or need any configuration to use it. From a configuration perspective, VLSM is simply a side effect of the ip address interface subcommand. Routers collectively configure VLSM by virtue of having IPv4 addresses in the same classful network but with different masks.
There are basically five steps to using VLSM. You must find all the subnet IDs that could be used, rule out the ones that would cause an overlap, and then check to see whether the requirement guides you to pick either the numerically lowest (or highest) subnet ID. This list outlines the specific steps:
Step 1. Pick the subnet mask (prefix length) for the new subnet, based on the design requirements (if not already listed as part of the question).
Step 2. Calculate all possible subnet numbers of the classful network using the mask from Step 1, along with the subnet broadcast addresses.
Step 3. Make a list of existing subnet IDs and matching subnet broadcast addresses.
Step 4. Rule out overlapping new subnets by comparing the lists from the previous two steps.
Step 5. Choose the new subnet ID from the remaining subnets identified at Step 4, paying attention to whether the question asks for the numerically lowest or numerically highest subnet ID.
Review Questions
Which of the following routing protocols support VLSM? (Choose three answers.)
RIPv1
RIPv2
EIGRP
OSPF
What does the acronym VLSM stand for?
Variable-length subnet mask
Very long subnet mask
Vociferous longitudinal subnet mask
Vector-length subnet mask
Vector loop subnet mask
R1 has configured interface Fa0/0 with the ip address 10.5.48.1 255.255.240.0 command. Which of the following subnets, when configured on another interface on R1, would not be considered an overlapping VLSM subnet?
10.5.0.0 255.255.240.0
10.4.0.0 255.254.0.0
10.5.32.0 255.255.224.0
10.5.0.0 255.255.128.0
R4 has a connected route for 172.16.8.0/22. Which of the following answers lists a subnet that overlaps with this subnet?
172.16.0.0/21
172.16.6.0/23
172.16.16.0/20
172.16.11.0/25
A design already includes subnets 192.168.1.0/26, 192.168.1.128/30, and 192.168.1.160/29. Which of the following subnets is the numerically lowest subnet ID that could be added to the design, if you wanted to add a subnet that uses a /28 mask?
192.168.1.144/28
192.168.1.112/28
192.168.1.64/28
192.168.1.80/28
192.168.1.96/28
Chapter Review
One key to doing well on the exams is to perform repetitive spaced review sessions. Review this chapter’s material using either the tools in the book, DVD, or interactive tools for the same material found on the book’s companion website. Refer to the “Your Study Plan” element for more details. Table 22-6 outlines the key review elements and where you can find them. To better track your study progress, record when you completed these activities in the second column.
Table 22-6 Chapter Review Tracking
Review Element |
Review Date(s) |
Resource Used |
Review key topics |
|
Book, DVD/website |
Review key terms |
|
Book, DVD/website |
Repeat DIKTA questions |
|
Book, PCPT |
Review memory tables |
|
Book, DVD/website |
Practice finding VLSM overlaps |
|
DVD Appendix H, DVD/website |
Practice adding new VLSM subnets |
|
DVD Appendix H, DVD/website |
Review All the Key Topics
Table 22-7 Key Topics for Chapter 22
Key Topic Element |
Description |
Page Number |
Table 22-1 |
Classless and classful routing protocols listed and compared |
520 |
Text |
Rule about subnetting designs cannot allow subnets to overlap |
521 |
List |
Steps to analyze an existing design to discover any VLSM overlaps |
523 |
List |
Steps to follow when adding a new subnet to an existing VLSM design |
525 |
Key Terms You Should Know
classful routing protocol
classless routing protocol
overlapping subnets
variable-length subnet masks (VLSM)
Additional Practice for This Chapter’s Processes
For additional practice with finding VLSM overlaps and adding a new subnet to a VLSM design, you may do the same set of practice problems using your choice of tools:
Application: Use the Variable-Length Subnet Masks application on the DVD or companion website.
PDF: Alternatively, practice the same problems found in both these apps using DVD Appendix H, “Practice for Chapter 22: Variable-Length Subnet Masks.”