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Using IP Unnumbered and VLSM


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Using IP Unnumbered and VLSM



Understanding IP Unnumbered



Configuring IP Unnumbered on Serial Interfaces



Understanding VLSM




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IP Routing Primer

From: IP Routing Primer
Author: Robert Wright
Publisher: Cisco Press (53)
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4. Using IP Unnumbered and VLSM

This chapter is broken into two sections. The first section discusses IP unnumbered. It provides scenarios showing how it works when properly configured and how it breaks when improperly configured.

The second section discusses variable length subnet masking (VLSM). It provides scenarios showing how it works when properly configured and how it breaks when improperly configured.

Before proceeding any further in this chapter, it is important to lay the groundwork for the discussion by defining the following terms:

  • IP unnumberedrefers to using the network or subnet address of a local LAN interface as the router's network or subnetwork address for a point-to-point serial link. The term point-to-point means that only two devices are on the link, as is the case on a T1 connection between two routers or a point-to-point subinterface to a Frame Relay network. Normally, a serial link has its own unique network or subnetwork address. IP unnumbered allows a network administrator to conserve network or subnetwork addresses. IP unnumbered is especially valuable for networks running IP routing protocols such as RIP V1 and IGRP, which do not support VLSM. Without VLSM support, a network must use the same subnet mask on its serial (WAN) interfaces as it does on its LAN interfaces. For a company using an 24-bit ( mask on a Class B address, this means applying a subnet capable of supporting 253 hosts on a WAN link that never has more than two hosts. See Chapter 8, “Hexadecimal and Binary Numbering and IP Addressing,” for more information on IP subnetting.

  • VLSM means applying different subnet masks to the same major net. With VLSM, a Class B address can be subnetted with a 253-host, 24-bit mask ( for the LAN subnets and a two-host, 30-bit mask ( for the WAN subnets.

  • Host routes are routes whose associated subnet mask is set to all 1s ( Remember that the subnet mask is used to separate the bits that are part of the network address from the bits that are part of the host address. In this case, the mask covers all of the bits, leaving no bits for the host portion. Because a subnet that is all network with no hosts would be of little use, it was decided that this subnet mask would be used as a host route pointing to a particular host. The section covering host routes in dial-on-demand routing (DDR) network configurations helps clarify this point.

  • DDR is providing temporary connections between networks or between users who are not based at the same site. DDR is often used when a permanent connection is not cost-efficient because data is only exchanged sporadically. Automated teller machines (ATMs) use DDR to contact their central host when verifying a transaction. This is typically done using a dial-up connection, such as a telephone line or an Integrated Services Digital Network (ISDN) connection.

Understanding IP Unnumbered

One of the reasons IP unnumbered is possible is that with only two hosts on any serial link—when the routers are at either end—there is no confusion about which device a packet is originating from or destined for. The following are a few rules that apply to using IP unnumbered:

  • The serial interface must be a point-to-point link.

  • You must use the same major network with the same mask on both sides of the WAN link.


  • You must use different major nets with no subnetting on both sides of the WAN link.

These rules are discussed in more detail in the following sections.

IP Unnumbered Causes Host Routes and Lost Connectivity

This section provides some background on IP unnumbered and some real-life scenarios of IP unnumbered in action. Figure4-1 represents the network topology used for most of this section.

Figure 4.1. Simplified network using IP unnumbered.>

In Table 4.1, the routing update from RouterC has a bit (1) set in the fourth octet (.1 = 00000001). RouterA compares this update with the mask of the IP address and the mask used by the unnumbered ethernet0 command. RouterA will discover that a bit is set outside the range of the mask, which must be interpreted as a host route.

Table 4-1. Binary example of an advertisement/mask mismatch.

Advertised mask = 10101000.01000111.00001000.00000001

Router's mask = 11111111.11111111.11111111.00000000

See the section on mask ambiguity in Chapter 3, “Discontiguous Networks, Summarization, and Subnet 0,” for more information on this issue.

Host Routes

As previously explained, ahost route refers to a route whose associated mask has all 32 bits set to 1— For an address and mask such as this, there can be only one host.

One common cause of host routes is a route advertisement that has bits set in the range that the receiving router interprets as outside of the subnet mask. In this case, the router compared the advertisement received from an adjacent router with the mask of the interface it was received over and found a mismatch. This is how RIP V1 and IGRP determine the mask to apply to advertisements they receive.

Table 4.1 shows an example of an advertisement that has a bit set to 1 in a region of the address not covered by the subnet mask in use.

Another common cause of host routes is a routing update explicitly advertised with a mask of Routing protocols such as OSPF and EIGRP include the masks associated with the routes in their routing updates.

Host routes are also often manually configured to resolve routing problems related to configuring dial-on-demand routing (DDR). The following section on DDR goes into more detail on this subject.

Hosts Routes Using DDR

Dial-up users may require host routes to ensure that they have full connectivity to the network. Although dial-up networking is beyond the scope of this book, this section attempts to clarify when host routes can be useful. In the scenario in Figure4-2, hosts A and B use modems to dial into the corporate network.

Figure 4.2. Async dial-up network using IP unnumbered without host routes.

In Figure4-2, RouterA is using IP unnumbered ethernet 0 on its Async interfaces. This results in multiple Async interfaces sharing the same subnet as a local Ethernet. However, the router needs to be able to keep track of the interfaces as separate subnets in order to route traffic correctly.

Remember that one of the rules of IP unnumbered is that it must be used only for point-to-point network connections. Extending the mask already applied to RouterA's Ethernet 0 interface when the addresses are configured on the asynchronous interfaces provides this point-to-point connection. Interface Async1 has an address of, and interface Async2 has an address of

When all of the hosts are connected to RouterA (they have dialed in to the network), they will have their own unique routes (their host routes) in RouterA's routing table. If RouterA receives a packet for from HostA, it does a longest match lookup of its routing table and determines that the best match is the subnet applied to its Ethernet 0 interface: It forwards the packet out of that interface.

On the other hand, if RouterA receives a packet for from HostB, the longest match lookup results in a match for the host route applied to Asynch1: The packet is forwarded out that interface to HostA (assuming either that the link to HostA is already up or that RouterA is configured to place a call to HostA when it receives a packet for it).

Without host routes being advertised by RouterA to RouterB for end systems A and B, RouterB assumes that an IP packet to or is for an end system on the Ethernet network connecting RouterA and RouterB. The gray box surrounding the (E0) interfaces for routers A and B in Figure4-2 shows where RouterB assumes that addresses and reside. RouterB ARPs (attempts to determine the layer two MAC address) for or directly instead of forwarding the packet to RouterA to reach end system A.


If you do not understand the role of the Address Resolution Protocol (ARP), refer to Chapter 7, “Bridging IP Between Dissimilar Media,” or RFC 826.

In Figure4-3, RouterA is advertising the host routes for end systems A and B. RouterB will do a longest match lookup for packets to either or and discover that the connected route to is not the longest match. RouterB will determine that these routes are reachable via its Ethernet 0 address, using RouterA as the next hop.

Figure 4.3. Async dial-up network using IP unnumbered without host routes.

The gray box surrounding HostA and HostB in Figure4-3 shows where RouterB will assume that addresses and reside now that the correct host routes are being advertised by RouterA.

This section has given you an intentionally simplistic example of DDR. DDR is an incredibly complex subject that can take up an entire book all by itself. Consult the documentation for the version of Cisco IOS you are using for more information on DDR internetworking. The next section explains how to configure IP unnumbered in non-DDR situations.


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