3.5 Router addressing issues

We discussed several addressing techniques that can be used together with router designs to improve overall performance and preserve bandwidth. These techniques included subnetting, hierarchical addressing (sometimes called route summarization), and supernetting (or CIDR). There are several other related techniques that are commonly implemented on routing platforms, including the following:

Unnumbered link support
Multinetting or multihoming
Broadcast forwarding (including BOOTP forwarding)

3.5.1 Unnumbered links

In large internetworks, there may be many serial point-to-point WAN links between routers, and each serial interface will have an IP address. Each link is effectively a private subnet, and this use of IP addresses has several potential drawbacks, including the following:

It wastes IP addresses. Since point-to-point links do not usually attach host devices, there is little point in wasting addresses and defining these subnets.
It becomes difficult to manage, as the network gets larger.
These point-to-point routes may show up in the routing table, making tables unnecessarily large and slowing down routing calculations and consequently delaying convergence.

A number of proprietary solutions to this problem have been implemented that enable serial interfaces to operate without any IP addresses assigned—a configuration usually referred to as unnumbered links. Modern routing protocols such as OSPF typically support the concept of unnumbered links as an optional feature.

Although generally considered to be extremely useful, there are some potential disadvantages of using unnumbered links in your design; these typically center around troubleshooting, as follows:

Ping won't be able to test reachability to either the local or remote serial interfaces.
ICMP Traceroute will lose track of the path over the serial line, since there is no IP address to trace.
Telnet will not work if you need to connect to the serial interface.

The standards (router requirements) suggest that the RouterID should be used as the remote target whenever a serial IP address is not available in this configuration. In reality, the ability to ping any real IP address or RouterID from a remote network is enough to prove that the serial interface is working, and it is usually possible to telnet either to the RouterID or a LAN interface remotely. The benefits of smaller, cleaner routing tables and simpler configuration management in my view outweigh any of these subsidiary issues.

3.5.2 Multinetting

Multinetting (sometimes called multihoming) was introduced largely to assist network managers in migrating from one IP addressing scheme to another. The concept is quite simple; each physical interface on a router has the ability to support a number of IP addresses of different classes and subnet masks. For example, in the following chart, interface el on the router is configured with multiple IP addresses.

Interface	Address	Mask
el	193.128.66.0	255.255.255.0
el	120.10.128.0	255.255.240.0
w1	140.40.0.0	255.255.0.0

In operational terms the router sees multiple network interfaces on the LAN port; the fact that they are all sitting on the same physical interface is of no real consequence. This configuration is sometimes referred to as a router on a stick, a lollipop, or a one-armed router. All communication between devices on the dual interface must go via the router. Clearly, this is inefficient if these devices are physically adjacent to each other and connected to the same piece of wire.

Typically, applications of multinetting might be migrating from an illegal network addressing scheme to a legal one. Suppose we had originally built our example network using the unregistered class B network address 120.10.0.0. On a single LAN port/single LAN port router we could still facilitate communications while migrating to a registered class C address. In this case the router would allow LAN-LAN communication transparently but would either filter out or NAT the illegal address when communicating with the wide area (if it were indeed a public network or attached elsewhere to a public network). Once the migration is complete, the LAN interface can be reconfigured by simply deleting the class B address and removing any filters or NAT rules. By this stage all LAN-LAN communication will be direct (avoiding the router).

3.5.3 Broadcast forwarding

Since routers are designed to discard broadcast destination addresses (e.g., 255.255.255.255) and particular source addresses (e.g., 0.0.0.0), applications and services that use such addresses are limited to use on the local network or subnetwork. For example, a client/server application that uses limited broadcasts cannot operate across a router. In most internetworks this is simply not practical, since clients will be widely distributed and the number of servers required in comparison would be quite low. Many such applications still exist; often they were originally intended for LAN use only, and operation over routers was never a design consideration. These applications are most commonly associated with device configuration or directory services (e.g., DNS, NIS, TFTP, and DHCP/BOOTP).

To resolve this problem, a third-party agent is required to transfer broadcast messages between clients and servers. Typically, this agent is implemented as a software feature on router platforms and called broadcast relay (sometimes called broadcast helper or broadcast gateway). This is not a routing function as such but a method of forwarding broadcasts between interfaces in a controlled manner. Because of the different ways in which protocol stacks operate, this feature is also stack specific; there are variants for IP, NetWare IPX, and AppleTalk, for example. In the case of individual services (such as BOOTP), additional packet manipulation is required, so there may be application-specific variations also. For further information on the BOOTP relay agents refer to [10].

3.5.4 Route summarization and CIDR

Two other design techniques, Classless Interdomain Routing Protocol (CIDR) and Route Summarization, are key for developing large-scale network designs.