The IP addressing hierarchy influences network routing. This section describes IP addressing hierarchy and how it reduces routing overhead. This section discusses the issues that influence the IP addressing plan and the routing protocol choice, including summarization, fixed-length subnet masking, variable-length subnet masking, and classful and classless routing protocols.

Hierarchical Addressing

The telephone numbering system is a hierarchical system. For example, the North American Numbering Plan includes the country code, the area code, the local exchange, and the line number. The telephone architecture has handled prefix routing, or routing based only on the prefix part of the address, for many years. For example, a telephone switch in Detroit, Michigan does not have to know how to reach a specific line in Portland, Oregon. It must simply recognize that the call is not local. A long-distance carrier must recognize that area code 503 is for Oregon, but it does not have to know the details of how to reach the specific line in Oregon.

The IP addressing scheme is also hierarchical, and prefix routing is not new in the IP environment either. As in the telephone example, IP routers make hierarchical decisions. Recall that an IP address comprises a prefix part and a host part. A router has to know only how to reach the next hop; it does not have to know the details of how to reach an end node that is not local. Routers use the prefix to determine the path for a destination address that is not local. The host part is used to reach local hosts.

Route Summarization

With route summarization, also referred to as route aggregation or supernetting, one route in the routing table represents many other routes. Summarizing routes reduces the routing update traffic (which can be important on low-speed links) and reduces the number of routes in the routing table and overall router overhead in the router receiving the routes. In a hierarchical network design, effective use of route summarization can limit the impact of topology changes to the routers in one section of the network.

IP Addressing Hierarchy Criteria

IP addressing hierarchy has an important impact on the routing protocol choice, and vice versa. The decision about how to implement the IP addressing hierarchy is usually based on the following questions:

■ Is hierarchy needed within the IP addressing plan?

■ What are the criteria for dividing the network into route summarization groups?

■ How is route summarization performed, and what is the correlation with routing?

■ Is a hierarchy of route summarization groups required?

■ How many end systems does each route summarization group or subgroup contain?

Benefits of Hierarchical Addressing

A network designer decides how to implement the IP addressing hierarchy based on the network’s size, geography, and topology. In large networks, hierarchy within the IP addressing plan is mandatory for a stable network (including stable routing tables). For the following reasons, a planned, hierarchical IP addressing structure, with room for growth, is recommended for networks of all sizes:

■ Influence of IP addressing on routing: An IP addressing plan influences the network’s overall routing. Before allocating blocks of IP addresses to various parts of the network and assigning IP addresses to devices, consider the criteria for an appropriate and effective IP addressing scheme. Routing stability, service availability, network scalability, and modularity are some crucial and preferred network characteristics and are directly affected by IP address allocation and deployment.

■ Modular design and scalable solutions: Whether building a new network or adding a new service on top of an existing infrastructure, a modular design helps to deliver a long-term, scalable solution. IP addressing modularity allows the aggregation of routing information on a hierarchical basis.

■ Route aggregation: Route aggregation is used to reduce routing overhead and improve routing stability and scalability. However, to implement route aggregation, a designer must be able to divide a network into contiguous IP address areas and must have a solid understanding of IP address assignment, route aggregation, and hierarchical routing.

Fixed- and Variable-Length Subnet Masks

Another consideration when designing the IP addressing hierarchy is the subnet mask to use— either the same mask for the entire major network or different masks for different parts of the major network. A major network is a Class A, B, or C network. Fixed-Length Subnet Masking (FLSM) is when all subnet masks in a major network must be the same. Variable-Length Subnet Masking (VLSM) is when subnet masks within a major network can be different. In modern networks, VLSM should be used to conserve the IP addresses.

For summarization to work correctly, the following requirements must be met:

■ Multiple IP addresses must share the same leftmost bits.

■ Routers must base their routing decisions on a 32-bit IP address and a prefix length of up to 32 bits.

■ Routing protocols must carry the prefix length with the 32-bit IP address.

For example, assume that a router has the following networks behind it:

192.168.168.0/24
192.168.169.0/24
192.168.170.0/24
192.168.171.0/24
192.168.172.0/24
192.168.173.0/24
192.168.174.0/24
192.168.175.0/24

Each of these networks could be advertised separately; however, this would mean advertising eight routes. Instead, this router can summarize the eight routes into one route and advertise 192.168.168.0/21. By advertising this one route, the router is saying, “Route packets to me if the destination has the first 21 bits the same as the first 21 bits of 192.168.168.0.”

Summarization Groups

To reduce the routing overhead in a large network, a multilevel hierarchy might be required. The depth of hierarchy depends on the network size and the size of the highest-level summarization group. Figure below shows an example of a network hierarchy.

Typical organization has up to three levels of hierarchy:

■ First level: Network locations typically represent the first level of hierarchy in enterprise networks. Each location typically represents a group of summarized subnets, known as a summarization group.

■ Second level: A second level of hierarchy can be done within first-level summarization groups. For example, a large location can be divided into smaller summarization groups that represent the buildings or cities within that location. Not all first-level summarization groups require a second level of hierarchy.

■ Third level: To further minimize the potential routing overhead and instability, a third level of hierarchy can exist within the second-level summarization group. For example, sections or floors within individual buildings can represent the third-level summarization group.

Fixed- and Variable-Length Subnet Masks

Another consideration when designing the IP addressing hierarchy is the subnet mask to use— either the same mask for the entire major network or different masks for different parts of the major network.

A major network is a Class A, B, or C network. Fixed-Length Subnet Masking (FLSM) is when all subnet masks in a major network must be the same. Variable-Length Subnet Masking (VLSM) is when subnet masks within a major network can be different. In modern networks, VLSM should be used to conserve the IP addresses.

Routing Protocol Considerations

To use VLSM, the routing protocol in use must be classless. Classful routing protocols permit only FLSM. With classful routing, routing updates do not carry the subnet mask. With classless routing, routing updates do carry the subnet mask.

Classful Routing Protocols

As illustrated at the top of Figure, the following rules apply when classful routing protocols are used:

■ The routing updates do not include subnet masks.

■ When a routing update is received and the routing information is about one of the following:

• Routes within the same major network as configured on the receiving interface, the subnet mask configured on the receiving interface is assumed to apply to the received routes also. Therefore, the mask must be the same for all subnets of a major network. In other words, subnetting must be done with FLSM.

• Routes in a different major network than configured on the receiving interface, the default major network mask is assumed to apply to the received routes. Therefore automatic route summarization is performed across major network (Class A, B, or C) boundaries, and subnetted networks must be contiguous.

Classless Routing Protocols

As illustrated in the lower portion of Figure below, the following rules apply when classless routing protocols are used:

■ The routing updates include subnet masks.

■ VLSM is supported.

■ Automatic route summarization at the major network boundary is not required, and route summarization can be manually configured.

■ Subnetted networks can be discontiguous.