Keys Points to Remember When Designing an IP Network

The following list of items should be addressed when preparing the IP addressing plan for your network:

• Identifying how many hosts and subnets will be required in the future requires communication with other departments in terms of the growth of personnel as well as the budget for network growth. Without the standard-issue crystal ball, a wider view must be taken at a high level to answer these questions, with the answers coming from a range of sources, including the senior management and executive team of the organization.

• The design of the IP network must take into consideration the network equipment and whether consideration should be given to different vendor equipment. Interoperability may well be an issue, particularly with some of the features offered by each product.

• For route aggregation (summarization) to occur, the address assignments must have topological significance.

• When using VLSM, the routing protocol must send the extended prefix (subnet mask) with the routing update.

• When using VLSM, the routing protocol must do a routing table lookup based on the longest match.

• Make certain that enough bits have been allowed for at each level of the hierarchical design to address all devices at that layer. Also be sure that growth of the network at each level has been anticipated. What address space is to be used (Class A, B, C, private, registered), and will it scale with the organization?

TIP Cisco offers many enhancements in its IOS. Most of these enhancements are interoperable. If they are not, they provide solutions for connecting to industry standards (which, of course, are fully supported by Cisco). Check with the Cisco web page ( to review the latest features and any connectivity issues.

In many cases, not enough consideration is given to IP address design with regard to the routing process, making a decision based on the longest address match. This is essential to the design of a VLSM network.

Consider a network as described in the preceding section, "Assigning IP VLSM Subnets for WAN Connections," using the Class B NIC address

The routing table has the following among its entries:

A packet comes into the router destined for the end host The router will forward to the network because the bit pattern matches the longest bit mask provided. The other routes are also valid, however, so the router has made a policy decision that it will always take the longest match.

This decision is based on the design assumption that has been made by the router that the longest match is directly connected to the router or that the network is out of the identified interface. If the end host actually resides on network, this network must be accessible through the interface that has learned of the subnet Summarization will have been configured because is an aggregate of various networks, including the network

If the network resides out of the interface that has learned about, then no traffic will ever reach this subnet because it will always forward based on the longest match in the routing table. The only solution is to turn off summarization and to list every subnet with the corresponding mask. If summarization is turned off, the subnet will not be summarized into the network It will consequently be the longest match in the routing table, and traffic will be sent to the destination network Figure 3-13 shows an example of route summarization.

Up to this point, this discussion has dealt with organizations that are designing an IP network for the first time. In reality, this is rarely the case unless a decision has been made to readdress the entire network.

Often the network has been up and running for some years. If this is the case, the usual task is to use some of the newer technologies now available to reduce and manage network traffic so that the network can grow without pain.

The simplest solution is to implement a more sophisticated routing protocol. Ideally a routing protocol that supports VLSM will be chosen and summarization will be enabled. However, it may not be possible to use the summarization feature. As explained earlier, this capability is determined in part by how well the addressing scheme mirrors and is supported by the physical topology.

Figure 3-13 Route Summarization and VLSM

IP datagram DA= SA=




Router A

Router A


Router A

routing table







IP datagram DA= SA= A A

Never receives any traffic as summarized into

IP datagram DA= SA=


The following guidelines can be used to determine whether summarization may be configured within a particular network:

• Does the network addressing reflect the physical topology of the network?

• Is the physical and logical topology of the network hierarchical?

• Given the network addressing scheme, do the addresses to be summarized share the same high-order bits?

• If the subnet addresses are clearly set on a single binary border, this suggests a prefix mask of /21 or Because the subnets are multiples of 8, they may be summarized by a higher subnet value that is divisible by 8, such as The following subnets provide an example:

• The nature of the traffic flow within the network should reflect the hierarchical logical and physical design.

• The routing protocol used must support VLSM.

Any design of a network requires very careful analysis of the current network and a clear understanding of the organization's plans. Unfortunately, it is not always possible to determine the nature or flow of data through a network. Intranets and internal web pages have made the nature of the traffic within an organization far more unpredictable.

The increased tendency for organizations to need flexibility or mobility in addressing can make the IP design very challenging. The design would need to include Dynamic Host Configuration Protocol (DHCP) and Domain Name System (DNS) servers to maximize the flexibility of the network.

It is also important to fully understand the nature of the traffic in the network, particularly if it is a client/server environment (in which the design must allow for servers to communicate with each other and with their clients).

It may not be possible to use the existing addressing of the organization. If this is the case, the decision must be made to readdress the network. The decision may be made either because the network cannot scale because of the limitations of the NIC number that has been acquired from the Internet, or because the original design did not allow for the current environment or growth.

If the addressing scheme is inadequate in size, you have several options. The first is to apply to the Internet for another address or to use private addressing.

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