Chapter 3: Medium-Sized Routed Network Construction

Cisco Press

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The default number of bits in the network ID is referred to as the classful prefix length. Therefore, a Class C address has a classful prefix length of /24, a Class B address has a classful prefix length of /16, and a Class A address has a classful prefix length of /8. This is illustrated in Figure 3-29.

Figure 3-29

Classful Prefix Length

The subnet address is created by taking address bits from the host-number portion of Class A, Class B, and Class C addresses. Usually a network administrator assigns the subnet address locally. Like IP addresses, each subnet address must be unique.

Each time one bit is borrowed from a host field, one less bit remains in the host field that can be used for host numbers, and the number of host addresses that can be assigned per subnet decreases by a power of 2.

When you borrow bits from the host field, note the number of additional subnets that are being created each time one more bit is borrowed. Borrowing two bits creates four possible subnets (22 = 4). Each time another bit is borrowed from the host field, the number of possible subnets increases by a power of 2, and the number of individual host addresses decreases by a power of 2.

The following are examples of how many subnets are available, based on the number of host bits that you borrow:

  • Using 3 bits for the subnet field results in 8 possible subnets (23 = 8).

  • Using 4 bits for the subnet field results in 16 possible subnets (24 = 16).

  • Using 5 bits for the subnet field results in 32 possible subnets (25 = 32).

  • Using 6 bits for the subnet field results in 64 possible subnets (26 = 64).

In general, you can use the following formula to calculate the number of usable subnets, given the number of subnet bits used:

Number of subnets = 2s (in which s is the number of subnet bits)

For example, you can subnet a network with a private network address of so that it provides 100 subnets and maximizes the number of host addresses for each subnet. The following list highlights the steps required to meet these needs:

  • How many bits will need to be borrowed?

  • 2s = 27 = 128 subnets (s = 7 bits)

  • What is the new subnet mask?

  • Borrowing 7 host bits = or /23

  • What are the first four subnets?

  •,,, and

  • What are the ranges of host addresses for the four subnets?


Introducing VLSMs

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 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 network address divided into subnets using /24 masking, and one of the subnetworks in this range,, further divided into smaller subnets with the /27 masking, as shown in Figure 3-30. These smaller subnets range from to In the figure, one of these smaller subnets,, is further divided with the /30 prefix, creating subnets with only two hosts to be used on the WAN links. The /30 subnets range from to In Figure 3-30, the WAN links used the,, and 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 summarizes all the addresses that are further subnets of, including those from subnet and from

As already discussed, with VLSMs, you can subnet an already subnetted address. Consider, for example, that you have a subnet address, 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 to give you more network addresses and fewer hosts per network. If, for example, you subnet to, you gain 64 (26) subnets, each of which could support 62 (26 – 2) hosts.

Figure 3-31 shows how subnet can be divided into smaller subnets.

03fig31 3-31

Calculating VLSM Networks

The following procedure shows how to further subnet to

Step 1

Write 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.)

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 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

To calculate the subnet addresses used on the WAN links, further subnet one of the unused /26 subnets. In this example, is further subnetted with a prefix of /30. This provides four more subnet bits, and therefore 16 (24) subnets for the WANs.

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,, is further subnetted for use on 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

The figure illustrates a summary route based on a full octet:,, and could be summarized into

Note - Router A can route to network, including all subnets of that network. However, if there were other subnets of elsewhere in the network (for example, if were discontiguous), summarizing in this way might not be valid. Discontiguous networks and summarization are discussed later in this chapter.

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 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:









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 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 and into a single update,

  • Router A receives three different routing updates but summarizes them into a single routing update before propagating it to the corporate network.

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.

  • 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 and to, packets addressed to would be routed through the 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 and subnets because RIPv1 cannot advertise subnets; both Router A and Router B advertise This leads to confusion when routing across network In this example, Router C receives routes about 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:

Note - Cisco IOS Software also provides an IP unnumbered feature that permits discontiguous subnets to be separated by an unnumbered link.

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