Based on this example, it would be fair to say that the router has four routes to the host. And clearly, the best route is the most specific host route. However, as noted before, it is impractical for every router to maintain information regarding each host in the network. Referring to the area-code model, it would be just as valid for a remote router to maintain the subnet or network routes—the path, or next hop, remains the same. Taken to the extreme, networks at the far end of a hub-and-spoke design, shown in Figure 3.2, can provide connectivity with a single route. The default route is used when no other routes match the packet. Since Router A in Figure 3.2 sees everything except 192.168.2.0 as being outside the serial inter-face, it is easy for the designer to omit all other routes from this router and, in essence, fully summarize the routing table.

FIGURE 3.2  The use of the default route in hub-and-spoke designs

The ODR (on-demand routing) protocol, discussed in Chapter 4, will present this concept in greater detail. ODR uses a default route on the remote router to forward packets accordingly.

Discontiguous Subnets

One of the problems frequently encountered with classful routing protocols is the need to support discontiguous subnets. A discontiguous subnet is two or more portions of a major network that are divided by another major network. Figure 3.3 illustrates the concept.

FIGURE 3.3  Discontiguous subnets

As shown, the major network 10.0.0.0 is split by the network 192.168.10.0. When running a classful routing protocol, RIP for example, each router believes that the major network is contained entirely outside its interface. Therefore, the router on the left believes that the entire 10.0.0.0 network is available outside the interface connected to the left. The same is true for the router on the right.

Administrators can resolve discontiguous subnet problems by using tunnels, or secondary interfaces, to link the two portions of the major network. This, in effect, makes the two networks contiguous. A better solution is to use a classless routing protocol that can summarize and accurately maintain information regarding the two halves of the network. This also avails VLSM and other features to the network and typically simplifies administration.

Discontiguous networks can be addressed with static mappings and other techniques; however, this can lead to black holes. This concept is presented in Chapter 13; briefly however, a black hole may leave a network unreachable under various failure scenarios.

Address Summarization

Address summarization provides a powerful function in IP networks. Under normal circumstances, each subnet would require a routing entry on every router in order to get packets to their destination. Thus, a collection of 32 subnets would require 32 routes on every router.

However, the router is concerned only with the path to the destination. As noted previously, a single default route could provide this path. While this configuration seriously limits redundancy and scalability in the network, it is a reasonable solution.

The compromise approach incorporates address summarization. Summarization can present hundreds of routes as a single entry in the routing table. This reduces memory demands and can prevent the need to recalculate a route should only a portion of the summarized network fail. For example, if 10.0.0.0 is available only via the FDDI (Fiber Distributed Date Interface) ring, it makes little difference if 10.12.24.0 is unavailable because the administrator shut down its interface.

Consider the following block of network addresses:

•  192.168.4.0

•  192.168.5.0

•  192.168.6.0

•  192.168.7.0

Each of these addresses would typically be deployed with the natural Class C mask—255.255.255.0. This would result in four route entries and four access-list entries. However, it would be much more efficient to use a single route entry and a single access list to represent all four address blocks.

Consider the binary representation of these addresses, as shown in Table 3.6.

Notice how the only variance in the addresses is limited to two bits, offset in bold? In order for the router to understand the range of addresses that is important, the administrator need only define the base address— 192.168.4.0—and the number of bits that are significant—22. The 23rd and 24th bits don’t matter, as whatever they equal still meets the range.

As a result of summarization, the network may be referenced as 192.168.4.0/22, or 255.255.252.0—the 23rd and 24th bits are moot. This summarization may be used in access lists (defined with a wildcard mask) or routing entries, although administrators should take care when using summarization and non-subnet-aware routing protocols. This topic will be discussed in detail in Chapter 4.

Summarization can be accomplished because the range of addresses meets two very important criteria. These are:

The range of addresses is a power of two. In this example, there are four addresses in the range.

•  The significant byte, which in this example is the third octet, is a multiple of the number of subnets in the range. Again, this number is four.

Consider summarization in a network’s design along with addressing. An addressing plan that places three subnets in each remote office will likely not summarize at all—192.168.3.0 through 192.168.5.255, for example. This leads to inefficiencies that are too important to ignore if the network is to scale, and as a result it is generally preferable to skip addresses in the assignment process so that each range provides for growth and evenness. It is not uncommon to assign eight 254-host networks to a fairly small office, although it is practical to do so only when using RFC 1918 address space.

Beyond the academic presentation of summarization, designers will find in subsequent chapters and their designs that summarization is imperative to the configuration of a hierarchical network. Without effective summarization, the network cannot scale and becomes difficult to administer.