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When an IP network is assigned more than one subnet mask for a given major network, it is
considered a network with VLSMs, overcoming the limitation of a fixed number of fixed-size
subnetworks imposed by a single subnet mask. Figure 3-30 shows the 172.16.0.0 network with
four separate subnet masks.
Figure 3-30 VLSM Network
VLSMs provide the capability to include more than one subnet mask within a network and the
capability to subnet an already subnetted network address. In addition, VLSM offers the following
benefits:
■ Even more efficient use of IP addresses: Without the use of VLSMs, companies must
implement a single subnet mask within an entire Class A, B, or C network number.
For example, consider the 172.16.0.0/16 network address divided into subnets using
/24 masking, and one of the subnetworks in this range, 172.16.14.0/24, further
divided into smaller subnets with the /27 masking, as shown in Figure 3-30. These
smaller subnets range from 172.16.14.0/27 to 172.16.14.224/27. In the figure, one
of these smaller subnets, 172.16.14.128/27, is further divided with the /30 prefix,
172.16.14.64/27
172.16.14.96/27
B
A
C
HQ
172.16.14.136/30
172.16.14.132/30
172.16.14.140/30
172.16.0.0/16
172.16.1.0/24
172.16.2.0/24
172.16.14.32/27
126 Chapter 3: Medium-Sized Routed Network Construction
creating subnets with only two hosts to be used on the WAN links. The /30 subnets
range from 172.16.14.128/30 to 172.16.14.156/30. In Figure 3-30, the WAN links
used the 172.16.14.132/30, 172.16.14.136/30, and 172.16.14.140/30 subnets out of
the range.
■ Greater capability to use route summarization: VLSM allows more hierarchical levels
within an addressing plan, allowing better route summarization within routing tables. For example,
in Figure 3-30, subnet 172.16.14.0/24 summarizes all the addresses that are further subnets of
172.16.14.0, including those from subnet 172.16.14.0/27 and from 172.16.14.128/30.
As already discussed, with VLSMs, you can subnet an already subnetted address. Consider, for
example, that you have a subnet address 172.16.32.0/20, and you need to assign addresses to a
network that has ten hosts. With this subnet address, however, you have more than 4000 (212 – 2
= 4094) host addresses, most of which will be wasted. With VLSMs, you can further subnet the
address 172.16.32.0/20 to give you more network addresses and fewer hosts per network. If, for
example, you subnet 172.16.32.0/20 to 172.16.32.0/26, you gain 64 (26) subnets, each of which
could support 62 (26 – 2) hosts.
Figure 3-31 shows how subnet 172.16.32.0/20 can be divided into smaller subnets.
Figure 3-31 Calculating VLSM Networks
The following procedure shows how to further subnet 172.16.32.0/20 to 172.16.32.0/26:
Step 1 Write 172.16.32.0 in binary form.
Step 2 Draw a vertical line between the twentieth and twenty-first bits, as shown
in Figure 3-31. (/20 was the original subnet boundary.)
1st subnet:
2nd subnet:
3rd subnet:
4th subnet:
5th subnet:
Network Subnet VLSM Host
Subnet
172 . 16
172 . 16
172 . 16
172 . 16
172 . 16
.0010
.0010
.0010
.0010
.0010
0000.00
0000.01
0000.10
0000.11
001.00
000000 =
000000 =
000000 =
000000 =
000000 =
172.16.32.0/26
172.16.32.64/26
172.16.32.128/26
172.16.32.192/26
172.16.33.0/26
Subnetted Address: 172.16.32.0/20
In Binary 10101100. 00010000.00100000.00000000
VLSM Address: 172.16.32.0/26
In Binary 10101100. 00010000.00100000.00000000
Implementing Variable-Length Subnet Masks 127
Step 3 Draw a vertical line between the twenty-sixth and twenty-seventh bits, as
shown in the figure. (The original /20 subnet boundary is extended six bits
to the right, becoming /26.)
Step 4 Calculate the 64 subnet addresses using the bits between the two vertical
lines, from lowest to highest in value. Figure 3-31 shows the first five
subnets available.
VLSMs are commonly used to maximize the number of possible addresses available for a
network. For example, because point-to-point serial lines require only two host addresses, using a
/30 subnet will not waste scarce IP addresses.
In Figure 3-32, the subnet addresses used on the Ethernets are those generated from subdividing
the 172.16.32.0/20 subnet into multiple /26 subnets. The figure illustrates where the subnet
addresses can be applied, depending on the number of host requirements. For example, the WAN
links use subnet addresses with a prefix of /30. This prefix allows for only two hosts: just enough
hosts for a point-to-point connection between a pair of routers.
Figure 3-32 VLSM Example
172.16.32.0/26
50 Hosts
172.16.32.64/26
50 Hosts
172.16.32.128/26
50 Hosts
172.16.32.192/26
50 Hosts
172.16.33.0/30
2 Hosts
172.16.33.4/30
2 Hosts
172.16.33.8/30
2 Hosts
172.16.33.12/30
2 Hosts
WAN Subnets
Derived from
172.16.33.0/26
LAN Subnets
Derived from
172.16.32.0/20
Entire Region Subnet
172.16.32.0/20
128 Chapter 3: Medium-Sized Routed Network Construction
To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26
subnets. In this example, 172.16.33.0/26 is further subnetted with a prefix of /30. This provides
four more subnet bits, and therefore 16 (24) subnets for the WANs.
Route Summarization with VLSM
In large internetworks, hundreds or even thousands of network addresses can exist. In these
environments, it is often not desirable for routers to maintain many routes in their routing table.
Route summarization, also called route aggregation or supernetting, can reduce the number of
routes that a router must maintain by representing a series of network numbers in a single
summary address. This section describes and provides examples of route summarization,
including implementation considerations.
Figure 3-33 shows that Router A can either send three routing update entries or summarize the
addresses into a single network number.
Figure 3-33 VLSM Route Summarization
NOTE Remember that only subnets that are unused can be further subnetted. In other words,
if you use any addresses from a subnet, that subnet cannot be further subnetted. In the example,
four subnet numbers are used on the LANs. Another unused subnet, 172.16.33.0/26, is further
subnetted for use on the WANs.
A
Routing Table
172.16.25.0/24
172.16.26.0/24
172.16.27.0/24
B
Routing Table
172.16.0.0/16
172.16.25.0/24
172.16.27.0/24
172.16.26.0/24
I can route to the
172.16.0.0/16 network.
Implementing Variable-Length Subnet Masks 129
The figure illustrates a summary route based on a full octet: 172.16.25.0/24, 172.16.26.0/24, and
172.16.27.0/24 could be summarized into 172.16.0.0/16.
Another advantage to using route summarization in a large, complex network is that it can isolate
topology changes from other routers. That is, if a specific link in the 172.16.27.0/24 domain were
“flapping,” or going up and down rapidly, the summary route would not change. Therefore, no
router external to the domain would need to keep modifying its routing table due to this flapping
activity. By summarizing addresses, you also reduce the amount of memory consumed by the
routing protocol for table entries.
Route summarization is most effective within a subnetted environment when the network
addresses are in contiguous blocks in powers of two. For example, 4, 16, or 512 addresses can be
represented by a single routing entry because summary masks are binary masks—just like subnet
masks—so summarization must take place on binary boundaries (powers of two).
Routing protocols summarize or aggregate routes based on shared network numbers within the
network. Classless routing protocols, such as RIP-2, OSPF, IS-IS, and EIGRP, support route
summarization based on subnet addresses, including VLSM addressing. Classful routing
protocols, such as RIP-1 and IGRP, automatically summarize routes on the classful network
boundary and do not support summarization on any other boundaries.
RFC 1518, “An Architecture for IP Address Allocation with CIDR,” describes summarization in
full detail.
Suppose a router receives updates for the following routes:
■ 172.16.168.0/24
■ 172.16.169.0/24
■ 172.16.170.0/24
■ 172.16.171.0/24
■ 172.16.172.0/24
NOTE Router A can route to network 172.16.0.0/16, including all subnets of that network.
However, if there were other subnets of 172.16.0.0 elsewhere in the network (for example, if
172.16.0.0 were discontiguous), summarizing in this way might not be valid. Discontiguous
networks and summarization are discussed later in this chapter.
130 Chapter 3: Medium-Sized Routed Network Construction
■ 172.16.173.0/24
■ 172.16.174.0/24
■ 172.16.175.0/24
To determine the summary route, the router determines the number of highest-order bits that
match in all the addresses. By converting the IP addresses to the binary format, as shown in Figure
3-34, you can determine the number of common bits shared among the IP addresses.
Figure 3-34 Summarizing Within an Octet
In Figure 3-34, the first 21 bits are in common among the IP addresses. Therefore, the best
summary route is 172.16.168.0/21. You can summarize addresses when the number of addresses
is a power of two. If the number of addresses is not a power of two, you can divide the addresses
into groups and summarize the groups separately.
To allow the router to aggregate the highest number of IP addresses into a single route summary,
your IP addressing plan should be hierarchical in nature. This approach is particularly important
when using VLSMs.
A VLSM design allows for maximum use of IP addresses, as well as more efficient routing update
communication when using hierarchical IP addressing. In Figure 3-35, for example, route
summarization occurs at two levels.
■ Router C summarizes two routing updates from networks 172.16.32.64/26 and
172.16.32.128/26 into a single update, 172.16.32.0/24.
■ Router A receives three different routing updates but summarizes them into a single routing
update before propagating it to the corporate network.
172.16.168.0/24 =
172.16.169.0/24 =
172.16.170.0/24 =
172.16.171.0/24 =
172.16.172.0/24 =
172.16.173.0/24 =
172.16.174.0/24 =
172.16.175.0/24 =
Number of Common Bits = 21
Summary: 172.16.168.0/21
Noncommon
Bits = 11
10101100
172
172
172
172
172
172
172
00010000
16
16
16
16
16
16
16
00000000
0
0
0
0
0
0
0
10101
10101
10101
10101
10101
10101
10101
10101
.
.
.
.
.
.
.
000
001
010
011
100
101
110
111
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Implementing Variable-Length Subnet Masks 131
Figure 3-35 Summarizing Addresses in a VLSM-Designed Network
Route summarization reduces memory use on routers and routing protocol network traffic.
Requirements for summarization to work correctly are as follows:
■ Multiple IP addresses must share the same highest-order bits.
■ Routing protocols must base their routing decisions on a 32-bit IP address and a prefix length
that can be up to 32 bits.
■ Routing protocols must carry the prefix length (subnet mask) with the 32-bit IP address.
Cisco routers manage route summarization in two ways:
■ Sending route summaries: Routing protocols, such as RIP and EIGRP, perform automatic
route summarization across network boundaries. Specifically, this automatic summarization
occurs for those routes whose classful network address differs from the major network
address of the interface to which the advertisement is being sent. For OSPF and IS-IS, you
must configure manual summarization. For EIGRP and RIP-2, you can disable automatic
route summarization and configure manual summarization. Whether routing summarization
is automatic or not depends on the routing protocol. It is recommended that you review the
documentation for your specific routing protocols. Route summarization is not always a
solution. You would not use route summarization if you needed to advertise all networks
across a boundary, such as when you have discontiguous networks.
172.16.128.0/20
172.16.64.0/20
172.16.32.128/26
172.16.32.64/26
172.16.64.0/20
C A
B
D
172.16.128.0/20
172.16.32.0/24 172.16.0.0/16
Corporate
Network
132 Chapter 3: Medium-Sized Routed Network Construction
■ Selecting routes from route summaries: If more than one entry in the routing table matches
a particular destination, the longest prefix match in the routing table is used. Several routes
might match one destination, but the longest matching prefix is used.
For example, if a routing table has different paths to 192.16.0.0/16 and to 192.16.5.0/24, packets
addressed to 192.16.5.99 would be routed through the 192.16.5.0/24 path because that address has
the longest match with the destination address.
Classful routing protocols summarize automatically at network boundaries. This behavior, which
cannot be changed with RIP-1 and IGRP, has important results, as follows:
■ Subnets are not advertised to a different major network.
■ Discontiguous subnets are not visible to each other.
In Figure 3-36, RIP-1 does not advertise the 172.16.5.0 255.255.255.0 and 172.16.6.0
255.255.255.0 subnets because RIPv1 cannot advertise subnets; both Router A and Router B
advertise 172.16.0.0. This leads to confusion when routing across network 192.168.14.0. In this
example, Router C receives routes about 172.16.0.0 from two different directions, so it cannot
make a correct routing decision.
Figure 3-36 Classful Summarization in Discontiguous Networks
You can resolve this situation by using RIP-2, OSPF, IS-IS, or EIGRP and not using
summarization because the subnet routes would be advertised with their actual subnet masks. For
example:
Summary of Implementing Variable-Length Subnet Masks
The following list summarizes the key points discussed in this section:
■ Subnetting lets you efficiently allocate addresses by taking one large broadcast domain and
breaking it up into smaller, more manageable broadcast domains.
NOTE Cisco IOS Software also provides an IP unnumbered feature that permits
discontiguous subnets to be separated by an unnumbered link.
C
RIPv1 will advertise
network 172.16.0.0.
B
RIPv1 will advertise
network 172.16.0.0.
172.16.5.0
255.255.255.0
192.168.14.16
255.255.255.240
172.16.6.0
255.255.255.0
A
Review Questions 133
■ VLSMs let you more efficiently allocate IP addresses by adding multiple layers of the
addressing hierarchy.
■ The benefits of route summarization include smaller routing tables and the ability to isolate
topology changes.
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