Variable-Length Subnet Masks

Variable-Length Subnet Masks
CHAPTER 22
Variable-Length Subnet Masks
This chapter covers the following exam topics:
1.0 Network Fundamentals
1.8 Configure, verify, and troubleshoot IPv4 addressing and subnetting
IPv4 addressing and subnetting use a lot of terms, a lot of small math steps, and a lot of
concepts that fit together. While learning those concepts, it helps to keep things as simple
as possible. One way this book has kept the discussion simpler so far was to show examples
that use one mask only inside a single Class A, B, or C network.
This chapter removes that restriction by introducing variable-length subnet masks (VLSM).
VLSM simply means that the subnet design uses more than one mask in the same classful
network. VLSM has some advantages and disadvantages, but when learning, the main challenge is that a subnetting design that uses VLSM requires more math, and it requires that
you think about some other issues as well. This chapter walks you through the concepts, the
issues, and the math.
“Do I Know This Already?” Quiz
Take the quiz (either here, or use the PCPT software) if you want to use the score to help
you decide how much time to spend on this chapter. The answers are at the bottom of the
page following the quiz, and the explanations are in DVD Appendix C and in the PCPT
software.
Table 22-1 “Do I Know This Already?” Foundation Topics Section-to-Question Mapping
Foundation Topics Section
Questions
VLSM Concepts and Configuration
1–2
Finding VLSM Overlaps
3–4
Adding a New Subnet to an Existing VLSM Design
5
1. Which of the following routing protocols support VLSM? (Choose three answers.)
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a.
RIPv1
b.
RIPv2
c.
EIGRP
d.
OSPF
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2. What does the acronym VLSM stand for?
a.
Variable-length subnet mask
b.
Very long subnet mask
c.
Vociferous longitudinal subnet mask
d.
Vector-length subnet mask
e.
Vector loop subnet mask
3. 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?
a.
10.5.0.0 255.255.240.0
b.
10.4.0.0 255.254.0.0
c.
10.5.32.0 255.255.224.0
d.
10.5.0.0 255.255.128.0
4. R4 has a connected route for 172.16.8.0/22. Which of the following answers lists a
subnet that overlaps with this subnet?
a.
172.16.0.0/21
b.
172.16.6.0/23
c.
172.16.16.0/20
d.
172.16.11.0/25
5. 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?
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a.
192.168.1.144/28
b.
192.168.1.112/28
c.
192.168.1.64/28
d.
192.168.1.80/28
e.
192.168.1.96/28
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Foundation Topics
VLSM Concepts and Configuration
VLSM occurs when an internetwork uses more than one mask for different subnets of a
single Class A, B, or C network. Figure 22-1 shows an example of VLSM used in Class A
network 10.0.0.0.
10.2.1.0 /24
10.2.2.0 /24
10.2.3.0 /24
10.2.4.0 /24
Albuquerque
10.1.4.0 /30
S0/1
S0/0
10.1.6.0 /30
S0/1
S0/0
Yosemite
Seville
10.3.4.0 /24
10.3.5.0 /24
10.3.6.0 /24
10.3.7.0 /24
10.1.1.0 /24
Figure 22-1
VLSM in Network 10.0.0.0: Masks /24 and /30
Figure 22-1 shows a typical choice of using a /30 prefix (mask 255.255.255.252) on point-topoint serial links, with mask /24 (255.255.255.0) on the LAN subnets. All subnets are of Class
A network 10.0.0.0, with two masks being used, therefore meeting the definition of VLSM.
Oddly enough, a common mistake occurs when people think that VLSM means “using more
than one mask in some internetwork” rather than “using more than one mask in a single
classful network.” For example, if in one internetwork diagram, all subnets of network
10.0.0.0 use a 255.255.240.0 mask, and all subnets of network 11.0.0.0 use a 255.255.255.0
mask, the design uses two different masks. However, Class A network 10.0.0.0 uses only
one mask, and Class A network 11.0.0.0 uses only one mask. In that case, the design does
not use VLSM.
VLSM provides many benefits for real networks, mainly related to how you allocate and
use your IP address space. Because a mask defines the size of the subnet (the number
of host addresses in the subnet), VLSM allows engineers to better match the need for
addresses with the size of the subnet. For example, for subnets that need fewer addresses,
the engineer uses a mask with fewer host bits, so the subnet has fewer host IP addresses.
This flexibility reduces the number of wasted IP addresses in each subnet. By wasting fewer
addresses, more space remains to allocate more subnets.
VLSM can be helpful for both public and private IP addresses, but the benefits are more
dramatic with public networks. With public networks, the address savings help engineers
avoid having to obtain another registered IP network number from regional IP address
assignment authorities. With private networks, as defined in RFC 1918, running out of
addresses is not as big a negative, because you can always grab another private network
from RFC 1918 if you run out.
Classless and Classful Routing Protocols
Before you can deploy a VLSM design, you must first use a routing protocol that supports
VLSM. To support VLSM, the routing protocol must advertise the mask along with each
subnet. Without mask information, the router receiving the update would be confused.
Answers to the “Do I Know This Already?” quiz:
1 B, C, D 2 A 3 A 4 D 5 C
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Chapter 22: Variable-Length Subnet Masks 531
For example, if a router learned a route for 10.1.8.0, but with no mask information, what
does that mean? Is that subnet 10.1.8.0/24? 10.1.8.0/23? 10.1.8.0/30? The dotted-decimal
number 10.1.8.0 happens to be a valid subnet number with a variety of masks, and because
multiple masks can be used with VLSM, the router has no good way to make an educated
guess. To effectively support VLSM, the routing protocol needs to advertise the correct
mask along with each subnet so that the receiving router knows the exact subnet that is
being advertised.
By definition, classless routing protocols advertise the mask with each advertised route,
and classful routing protocols do not. The classless routing protocols, as noted in Table
22-2, are the newer, more advanced routing protocols. Not only do these more advanced
classless routing protocols support VLSM, but they also support manual route summarization, which allows a routing protocol to advertise one route for a larger subnet instead of
multiple routes for smaller subnets.
Table 22-2
Classless and Classful Interior IP Routing Protocols
Routing
Protocol
Is It
Classless?
Sends Mask
in Updates?
Supports
VLSM?
Supports Manual Route
Summarization?
RIPv1
No
No
No
No
RIPv2
Yes
Yes
Yes
Yes
EIGRP
Yes
Yes
Yes
Yes
OSPF
Yes
Yes
Yes
Yes
Beyond VLSM itself, the routing protocols do not have to be configured to support VLSM
or to be classless. There is no command to enable or disable the fact that classless routing
protocols include the mask with each route. The only configuration choice you must make
is to use a classless routing protocol.
VLSM Configuration and Verification
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 using the ip
address interface subcommand. Routers collectively configure VLSM by virtue of having IP
addresses in the same classful network but with different masks.
22
For example, Example 22-1 shows two of the interfaces from router Yosemite from Figure
22-1. The example shows the IP address assignments on two interfaces, one with a /24 mask
and one with a /30 mask, both with IP addresses in Class A network 10.0.0.0.
Example 22-1 Configuring Two Interfaces on Yosemite, Resulting in VLSM
Yosemite# configure terminal
Yosemite(config)# interface Fa0/0
Yosemite(config-if)# ip address 10.2.1.1 255.255.255.0
Yosemite(config-if)# interface S0/1
Yosemite(config-if)# ip address 10.1.4.1 255.255.255.252
The use of VLSM can also be detected by a detailed look at the output of the show ip
route command. This command lists routes in groups, by classful network, so that you see
all the subnets of a single Class A, B, or C network all in a row. Just look down the list, and
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look to see, if any, how many different masks are listed. For example, Example 22-2 lists
the routing table on Albuquerque from Figure 22-1; Albuquerque uses masks /24 and /30
inside network 10.0.0.0, as noted in the highlighted line in the example.
Example 22-2 Albuquerque Routing Table with VLSM
Albuquerque# show ip route
! Legend omitted for brevity
10.0.0.0/8 is variably subnetted, 14 subnets, 3 masks
D
10.2.1.0/24 [90/2172416] via 10.1.4.1, 00:00:34, Serial0/0
D
10.2.2.0/24 [90/2172416] via 10.1.4.1, 00:00:34, Serial0/0
D
10.2.3.0/24 [90/2172416] via 10.1.4.1, 00:00:34, Serial0/0
D
10.2.4.0/24 [90/2172416] via 10.1.4.1, 00:00:34, Serial0/0
D
10.3.4.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1
D
10.3.5.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1
D
10.3.6.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1
D
10.3.7.0/24 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1
C
10.1.1.0/24 is directly connected, FastEthernet0/0
L
10.1.1.1/32 is directly connected, FastEthernet0/0
C
10.1.6.0/30 is directly connected, Serial0/1
L
10.1.6.1/32 is directly connected, Serial0/1
C
10.1.4.0/30 is directly connected, Serial0/0
L
10.1.4.1/32 is directly connected, Serial0/0
NOTE For the purposes of understanding whether a design uses VLSM, ignore the /32
“local” routes that a router automatically creates for its own interface IP addresses.
So ends the discussion of VLSM as an end to itself. This chapter is devoted to VLSM, but it
took a mere three to four pages to fully describe it. Why the entire VLSM chapter? Well,
to work with VLSM, to find problems with it, to add subnets to an existing design, and to
design using VLSM from scratch—in other words, to apply VLSM to real networks—takes
skill and practice. To do these same tasks on the exam requires skill and practice. The rest
of this chapter examines the skills to apply VLSM and provides some practice for these two
key areas:
■
Finding VLSM overlaps
■
Adding new VLSM subnets without overlaps
Finding VLSM Overlaps
Regardless of whether a design uses VLSM, the subnets used in any IP internetwork design
should not overlap their address ranges. When subnets in different locations overlap their
addresses, a router’s routing table entries overlap. As a result, hosts in different locations
can be assigned the same IP address. Routers clearly cannot route packets correctly in
these cases. In short, a design that uses overlapping subnets is considered to be an incorrect
design and should not be used.
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Chapter 22: Variable-Length Subnet Masks 533
This section begins with a short discussion about VLSM design, to drive home the ideas
behind VLSM overlaps. It then gets into an operational and troubleshooting approach to
the topic, by looking at existing designs and trying to find any existing overlaps.
Designing Subnetting Plans with VLSM
When creating a subnetting plan using VLSM, you have to be much more careful in choosing what subnets to use. First, whatever masks you use in a VLSM design, each subnet ID
must be a valid subnet ID given the mask that you use for that subnet.
For example, consider a subnet plan for Class B network 172.16.0.0. To create a subnet with
a /24 mask, the subnet ID must be a subnet ID that you could choose if you subnetted the
whole Class B network with that same mask. Chapter 21, “Subnet Design,” discusses how to
find those subnets in depth, but with a Class B network and a /24 mask, the possible subnet IDs should be easy to calculate by now: 172.16.0.0 (the zero subnet), then 172.16.1.0,
172.16.2.0, 172.16.3.0, 172.16.4.0, and so on, up through 172.16.255.0.
NOTE Subnet IDs must always follow this important binary rule as noted back in Chapter
16, “Analyzing Existing Subnets”: In binary, each subnet ID has a host field of all binary 0s.
If you use the math and processes to find all subnet IDs per Chapter 21, all those subnet
IDs happen to have binary 0s in the host fields.
Now expand your thinking about subnet IDs to a VLSM design. To begin, you would
decide that you need some subnets with one mask, other subnets with another mask, and so
on, to meet the requirements for different sizes of different subnets. For instance, imagine
you start with a brand-new VLSM design, with Class B network 172.16.0.0. You plan to
have some subnets with /22 masks, some with /23, and some with /24. You might develop
then a planning diagram, or at least draw the ideas, with something like Figure 22-2.
List of /22 Subnets
List of /23 Subnets
List of /24 Subnets
172.16.0.0 /22
172.16.0.0 /23
172.16.0.0 /24
22
172.16.1.0 /24
172.16.2.0 /23
172.16.2.0 /24
172.16.3.0 /24
172.16.4.0 /22
172.16.4.0 /23
172.16.4.0 /24
172.16.5.0 /24
172.16.6.0 /23
.
.
.
Figure 22-2
.
.
.
172.16.6.0 /24
172.16.7.0 /24
.
.
.
Possible Subnet IDs of Network 172.16.0.0, with /22, /23, and /24 Masks
The drawing shows the first few subnet IDs available with each mask, but you cannot use
all subnets from all three lists in a design. As soon as you choose to use one subnet from
any column, you remove some subnets from the other lists because subnets cannot overlap. Overlapping subnets are subnets whose range of addresses include some of the same
addresses.
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As an example, Figure 22-3 shows the same list of the first few possible /22, /23, and /24
subnets of Class B network 172.16.0.0. However, it shows a check mark beside two subnets
that have been allocated for use; that is, on paper, the person making the subnetting plan
has decided to use these two subnets somewhere in the network. The subnets with a dark
gray shading and an X in them can no longer be used because they have some overlapping
addresses with the subnets that have check marks (172.16.3.0/24 and 172.16.4.0/22).
List of /22 Subnets
List of /23 Subnets
List of /24 Subnets
172.16.0.0 /22
172.16.0.0 /23
172.16.0.0 /24
172.16.1.0 /24
172.16.2.0 /23
172.16.2.0 /24
172.16.3.0 /24
172.16.4.0 /22
172.16.4.0 /23
172.16.4.0 /24
172.16.5.0 /24
172.16.6.0 /23
.
.
.
Figure 22-3
.
.
.
172.16.6.0 /24
172.16.7.0 /24
.
.
.
Selecting Two Subnets Disallows Other Subnets in Different Columns
Just to complete the example, first look at subnet 172.16.4.0 on the lower left. That subnet
includes addresses from the subnet ID of 172.16.4.0 through the subnet broadcast address
of 172.16.7.255. As you can see just by looking at the subnet IDs to the right, all the subnets referenced with the arrowed lines are within that same range of addresses.
Now look to the upper right of the figure, to subnet 172.16.3.0/24. The subnet has a range
of 172.16.3.0–172.16.3.255 including the subnet ID and subnet broadcast address. That subnet overlaps with the two subnets referenced to the left. For instance, subnet 172.16.0.0/22
includes the range from 172.16.0.0–172.16.3.255. But because there is some overlap, once
the design has allocated the 172.16.3.0/24 subnet, the 172.16.2.0/23 and 172.16.0.0/22 subnets could not be used without causing problems, because:
A subnetting design, whether using VLSM or not, should not allow subnets whose address
ranges overlap. If overlapping subnets are implemented, routing problems occur and some
hosts simply cannot communicate outside their subnets.
These address overlaps are easier to see when not using VLSM. When not using VLSM,
overlapped subnets have identical subnet IDs, so to find overlaps, you just have to look at
the subnet IDs. With VLSM, overlapped subnets may not have the same subnet ID, as was
the case in this most recent example with the subnets across the top of Figure 22-3. To
find these overlaps, you have to look at the entire range of addresses in each subnet, from
subnet ID to subnet broadcast address, and compare the range to the other subnets in the
design.
An Example of Finding a VLSM Overlap
For example, imagine that a practice question for the CCENT exam shows Figure 22-4.
It uses a single Class B network (172.16.0.0), with VLSM, because it uses three different
masks: /23, /24, and /30.
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Chapter 22: Variable-Length Subnet Masks 535
172.16.4.1 /23
Address Range?
Address Range?
172.16.2.1 /23
Fa0/0
172.16.9.1 /30
S0/0/1
S0/0/1
172.16.9.2 /30
S0/1/0
172.16.9.5 /30
172.16.9.6 /30
S0/0/1
R2
Fa0/0
Address Range?
R1
Address Range?
Fa0/0
R3
Address Range?
Figure 22-4
172.16.5.1 /24
VLSM Design with Possible Overlap
Now imagine that the exam question shows you the figure, and either directly or indirectly
asks whether overlapping subnets exist. This type of question might simply tell you that
some hosts cannot ping each other, or it might not even mention that the root cause could
be that some of the subnets overlap. To answer such a question, you could follow this simple but possibly laborious process:
Step 1.
Calculate the subnet ID and subnet broadcast address of each subnet, which
gives you the range of addresses in that subnet.
Step 2.
List the subnet IDs in numerical order (along with their subnet broadcast
addresses).
Step 3.
Scan the list from top to bottom, comparing each pair of adjacent entries, to
see whether their range of addresses overlaps.
For example, Table 22-3 completes the first two steps based on Figure 22-4, listing the subnet IDs and subnet broadcast addresses, in numerical order based on the subnet IDs.
Table 22-3 Subnet IDs and Broadcast Addresses, in Numerical Order, from Figure 22-4
Subnet
Subnet Number
Broadcast Address
R1 LAN
172.16.2.0
172.16.3.255
R2 LAN
172.16.4.0
172.16.5.255
R3 LAN
172.16.5.0
172.16.5.255
R1-R2 serial
172.16.9.0
172.16.9.3
R1-R3 serial
172.16.9.4
172.16.9.7
22
The VLSM design is invalid in this case because of the overlap between R2’s LAN subnet
and R3’s LAN subnet. As for the process, Step 3 states the somewhat obvious step of comparing the address ranges to see whether any overlaps occur. Note that, in this case, none
of the subnet numbers are identical, but two entries (highlighted) do overlap. The design is
invalid because of the overlap, and one of these two subnets would need to be changed.
As far as the three-step process works, note that if two adjacent entries in the list overlap,
compare three entries at the next step. The two subnets already marked as overlapped can
overlap with the next subnet in the list. For example, the three subnets in the following list
overlap in that the first subnet overlaps with the second and third subnets in the list. If you
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followed the process shown here, you would have first noticed the overlap between the
first two subnets in the list, so you would then also need to check the next subnet in the list
to find out if it overlapped.
10.1.0.0/16 (subnet ID 10.1.0.0, broadcast 10.1.255.255)
10.1.200.0/24 (subnet ID 10.1.200.0, broadcast 10.1.200.255)
10.1.250.0/24 (subnet ID 10.1.250.0, broadcast 10.1.250.255)
Practice Finding VLSM Overlaps
As typical of anything to with applying IP addressing and subnetting, practice helps. To that
end, Table 22-4 lists three practice problems. Just start with the five IP addresses listed in
a single column, and then follow the three-step process outlined in the previous section to
find any VLSM overlaps. The answers can be found near the end of this chapter, in the section “Answers to Earlier Practice Problems.”
Table 22-4
VLSM Overlap Practice Problems
Problem 1
Problem 2
Problem 3
10.1.34.9/22
172.16.126.151/22
192.168.1.253/30
10.1.29.101/23
172.16.122.57/27
192.168.1.113/28
10.1.23.254/22
172.16.122.33/30
192.168.1.245/29
10.1.17.1/21
172.16.122.1/30
192.168.1.125/30
10.1.1.1/20
172.16.128.151/20
192.168.1.122/30
Adding a New Subnet to an Existing VLSM Design
The task described in this section happens frequently in real networks: choosing new subnets to add to an existing design. In real life, you can use IP Address Management (IPAM)
tools that help you choose a new subnet so that you do not cause an overlap. However, for
both real life and for the CCENT and CCNA Routing and Switching exams, you need to
be ready to do the mental process and math of choosing a subnet that does not create an
overlapped VLSM subnet condition. In other words, you need to pick a new subnet and not
make a mistake!
For example, consider the internetwork shown earlier in Figure 22-2, with classful network
172.16.0.0. An exam question might suggest that a new subnet, with a /23 prefix length,
needs to be added to the design. The question might also say, “Pick the numerically lowest
subnet number that can be used for the new subnet.” In other words, if both 172.16.4.0 and
172.16.6.0 would work, use 172.16.4.0.
So, you really have a couple of tasks: To 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 question 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.
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Chapter 22: Variable-Length Subnet Masks 537
Step 3.
Make a list of existing subnet IDs and matching subnet broadcast addresses.
Step 4.
Compare the existing subnets to the candidate new subnets to rule out overlapping new subnets.
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.
An Example of Adding a New VLSM Subnet
For example, Figure 22-5 shows an existing internetwork that uses VLSM. (The figure uses
the same IP addresses as shown in Figure 22-4, but with R3’s LAN IP address changed to fix
the VLSM overlap shown in Figure 22-4.) In this case, you need to add a new subnet to support 300 hosts. Imagine that the question tells you to use the smallest subnet (least number
of hosts) to meet that requirement. You use some math and logic you learned earlier in your
study to choose mask /23, which gives you 9 host bits, for 29 – 2 = 510 hosts in the subnet.
172.16.4.1 /23
172.16.2.1 /23
Fa0/0
172.16.9.1 /30
S0/0/1
S0/0/1
172.16.9.2 /30
S0/1/0
172.16.9.5 /30
172.16.9.6 /30
S0/0/1
R2
Fa0/0
R1
Fa0/0
R3
Figure 22-5
172.16.5.1 /24
Internetwork to Which You Need to Add a /23 Subnet, Network 172.16.0.0
At this point, just follow the steps listed before Figure 22-5. For Step 1, you have already
been given the mask (/23). For Step 2, you need to list all the subnet numbers and broadcast
addresses of 172.16.0.0, assuming the /23 mask. You will not use all these subnets, but you
need the list for comparison to the existing subnets. Table 22-5 shows the results, at least
for the first five possible /23 subnets.
Table 22-5
Subnet
22
First Five Possible /23 Subnets
Subnet Number
Subnet Broadcast Address
First (zero)
172.16.0.0
172.16.1.255
Second
172.16.2.0
172.16.3.255
Third
172.16.4.0
172.16.5.255
Fourth
172.16.6.0
172.16.7.255
Fifth
172.16.8.0
172.16.9.255
Next, at Step 3, list the existing subnet numbers and broadcast addresses, as shown earlier in
Figure 22-5. To do so, do the usual math to take an IP address/mask to then find the subnet
ID and subnet broadcast address. Table 22-6 summarizes that information, including the
locations, subnet numbers, and subnet broadcast addresses.
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Table 22-6 Existing Subnet IDs and Broadcast Addresses from Figure 22-5
Subnet
Subnet Number
Subnet Broadcast Address
R1 LAN
172.16.2.0
172.16.3.255
R2 LAN
172.16.4.0
172.16.5.255
R3 LAN
172.16.6.0
172.16.6.255
R1-R2 serial
172.16.9.0
172.16.9.3
R1-R3 serial
172.16.9.4
172.16.9.7
At this point, you have all the information you need to look for the overlap at Step 4.
Simply compare the range of numbers for the subnets in the previous two tables. Which of
the possible new /23 subnets (Table 22-5) overlap with the existing subnets (Table 22-6)? In
this case, the second through fifth subnets in Table 22-5 overlap, so rule those out as candidates to be used. (Table 22-5 denotes those subnets with gray highlights.)
Step 5 has more to do with the exam than with real network design, but it is still worth listing as a separate step. Multiple-choice questions sometimes need to force you into a single
answer, and asking for the numerically lowest or highest subnet does that. This particular
example asks for the numerically lowest subnet number, which in this case is 172.16.0.0/23.
NOTE The answer, 172.16.0.0/23, happens to be a zero subnet. For the exam, the zero
subnet should be avoided if (a) the question implies the use of classful routing protocols or
(b) the routers are configured with the no ip subnet-zero global configuration command.
Otherwise, assume that the zero subnet can be used.
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 22-7 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 22-7 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
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Chapter 22: Variable-Length Subnet Masks 539
Review All the Key Topics
Table 22-8 Key Topics for Chapter 22
Key Topic
Element
Description
Page
Number
Table 22-2
Classless and classful routing protocols listed and compared
531
Text
Rule about subnetting designs cannot allow subnets to overlap
532
List
Steps to analyze an existing design to discover any VLSM
overlaps
535
List
Steps to follow when adding a new subnet to an existing VLSM
design
536
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.”
Answers to Earlier Practice Problems
22
Answers to Practice Finding VLSM Overlaps
This section lists the answers to the three practice problems in the section “Practice Finding
VLSM Overlaps,” as listed earlier in Table 22-4. Note that the tables that list details of the
answer reordered the subnets as part of the process.
In Problem 1, the second and third subnet IDs listed in Table 22-9 happen to overlap. The
second subnet’s range completely includes the range of addresses in the third subnet.
Table 22-9 VLSM Overlap Problem 1 Answers (Overlaps Highlighted)
Reference
Original Address and Mask
Subnet ID
Broadcast Address
1
10.1.1.1/20
10.1.0.0
10.1.15.255
2
10.1.17.1/21
10.1.16.0
10.1.23.255
3
10.1.23.254/22
10.1.20.0
10.1.23.255
4
10.1.29.101/23
10.1.28.0
10.1.29.255
5
10.1.34.9/22
10.1.32.0
10.1.35.255
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540
CCENT/CCNA ICND1 100-105 Official Cert Guide
In Problem 2, again the second and third subnet IDs (listed in Table 22-10) happen to overlap, and again, the second subnet’s range completely includes the range of addresses in the
third subnet. Also, the second and third subnet IDs are the same value, so the overlap is
more obvious.
Table 22-10 VLSM Overlap Problem 2 Answers (Overlaps Highlighted)
Reference
Original Address and Mask
Subnet ID
Broadcast Address
1
172.16.122.1/30
172.16.122.0
172.16.122.3
2
172.16.122.57/27
172.16.122.32 172.16.122.63
3
172.16.122.33/30
172.16.122.32 172.16.122.35
4
172.16.126.151/22
172.16.124.0
172.16.127.255
5
172.16.128.151/20
172.16.128.0
172.16.143.255
In Problem 3, three subnets overlap. Subnet 1’s range completely includes the range of
addresses in the second and third subnets, as shown in Table 22-11. Note that the second
and third subnets do not overlap with each other, so for the process in this book to find all
the overlaps, after you find that the first two subnets overlap, you should compare the next
entry in the table (3) with both of the two known-to-overlap entries (1 and 2).
Table 22-11 VLSM Overlap Problem 3 Answers (Overlaps Highlighted)
Reference Original Address and Mask
Subnet ID
1
192.168.1.113/28
192.168.1.112 192.168.1.127
2
192.168.1.122/30
192.168.1.120 192.168.1.123
3
192.168.1.125/30
192.168.1.124 192.168.1.127
4
192.168.1.245/29
192.168.1.240 192.168.1.247
5
192.168.1.253/30
192.168.1.252 192.168.1.255
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Broadcast Address
4/12/16 4:28 PM
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