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Extending IP Addresses Using VLSMs (Variable- Length Subnet Masks)


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Extending IP Addresses Using VLSMs (Variable- Length Subnet Masks)



IP Address Issues and Solutions



Variable-Length Subnet Masks



Route Summarization Overview



Other Addressing Considerations




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Advanced Cisco Router Configuration

From: Advanced Cisco Router Configuration
Author: Systems Cisco; Laura Chappell
Publisher: Cisco Press (53)
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7. Extending IP Addresses Using VLSMs (Variable-Length Subnet Masks)

This chapter discusses the key components related to IP addressing, including variable- length subnet masks and route summarization. The content in this chapter is a prerequisite to understanding how to reduce routing table entries and the amount of route updates issued by routing protocols, such as OSPF (Open Shortest Path First). Knowing these IP addressing techniques enables you to define an appropriate IP address scheme for your network.

IP Address Issues and Solutions

When IP addressing was first defined in 1981, it was designed as a 32-bit number that had two components: a network address and a node (host) address. Classes of addresses were also defined: class A, B, and C, and later classes D and E. Since then, the growth of the Internet has been incredible. Following are two addressing issues that have resulted from this explosion:

  • IP address exhaustion—This is due largely to the random allocation of IP addresses by the NIC (Network Information Center). It is also due to the fact that not all IP classes are suitable for a typical network topology, as you will see later in this chapter.

  • Routing table growth and manageability—One source indicates that in 1990 only about 5,000 routes needed to be tracked in order to use the Internet. By 1995, this number had grown to 35,000 routes. In addition to the exponential growth of the Internet, the random assignment of IP addresses throughout the world has also contributed to the exponential growth of routing tables.

IPv6 (the next-generation IP) responds to these problems by introducing a 128-bit address. In the meantime, RFCs (Requests for Comment) have been introduced to enable the current IP addressing scheme to be organized in a hierarchical manner. One particularly effective method of combating these problems is by using addressing hierarchies, as described in the next section.

Using Addressing Hierarchies

What is an addressing hierarchy, and why do you want to have it?

Perhaps the best known addressing hierarchy is the telephone network. The telephone network uses a hierarchical numbering scheme that includes country codes, area codes, and local exchange numbers, as shown in Figure 7-1. For example, if you are in San Jose, California, and call someone else in San Jose, then you dial the San Jose local exchange number, 528, and the person's telephone number, 7777. The central office, upon seeing the number 528, recognizes that the destination telephone is within its area so it looks for number 7777 and transfers the call.

Figure 7-1. Telephone number hierarchy.

To call Aunt Judy in Alexandria, Virginia, from San Jose, dial the area code, 703, the Alexandria prefix, 555, and then Aunt Judy's local number, 1212. The central office first looks up number 703 and determines that it is not in its local area. The central office immediately routes the call to a central office in Alexandria. The San Jose central office does not know where 555-1212 is, nor does it have to. It only needs to know the area codes, which summarize the local telephone numbers within an area.

If there were no hierarchical structure, every central office would need to have every telephone number, worldwide, in its locator table. With a simple hierarchical addressing scheme, the central office uses country codes and area codes to determine how to route a call to its destination. A summary number (address) represents a group of numbers. For example, an area code, such as 408, is a summary number for the San Jose area. That is, if you dial 408 from anywhere in the United States, and then a seven-digit telephone number, the central office will route the call to a San Jose central office. This is the kind of addressing strategy that the Internet gurus are trying to work toward, and that you as a network administrator should implement in your own internetwork.

The benefits of hierarchical addressing are twofold:

  • Efficient allocation of addresses—Hierarchical addressing enables one to optimize the use of the available addresses. because you group them contiguously. With random address assignment, you may end up wasting groups of addresses because of addressing conflicts.

  • Reduced number of routing table entries—Whether it is with your Internet routers, or your internal routers, you should try to keep your routing tables as small as possible by using route summarization. Route summarization is a way of having a single IP address represent a collection of IP addresses when you employ a hierarchical addressing plan. By summarizing routes, you can keep your routing table entries manageable, which means the following:

    • More efficient routing

    • Reduced number of CPU cycles when recalculating a routing table, or when sorting through the routing table entries to find a match

    • Reduced router memory requirements

Slowing IP Address Depletion

Since the 1980s, several solutions have been developed to slow the depletion of IP addresses and to reduce the number of Internet route table entries by enabling more hierarchical layers in an IP address. The solutions discussed in this chapter are as follows:

  • Subnet MaskingRFC 950 (1985); 1812 (1995). Developed to add another level of hierarchy to an IP address. This additional level allows for extending the number of network addresses derived from a single IP address. (Discussed in Introduction to Cisco Router Configuration, ISBN: 1-57870-076-0, by Cisco Press.)

  • Variable-Length Subnet MasksRFC 1009 (1987). Developed to allow the network designer to utilize multiple address schemes within a given class of address. This strategy can be used only when it is supported by the routing protocol, such as OSPF and EIGRP.

  • Address Allocation for Private Internets—RFC 1918 (1996). Developed for organizations that do not need much access to the Internet. The only reason to have a NIC-assigned IP address is to interconnect to the Internet. Any and all companies can use the privately assigned IP addresses within the organization, rather than using a NIC-assigned IP address unnecessarily.

  • Network Address Translation—RFC 1631 (1994). Developed for those companies that use private addressing or use non-NIC-assigned IP addresses. This strategy enables an organization to access the Internet with a NIC- assigned address without having to reassign the private or “illegal” addresses that are already in place.

  • Classless Inter-Domain Routing (CIDR)RFCs 1518 and 1519 (1993). This is another method used for and developed for ISPs. This strategy suggests that the remaining IP addresses be allocated to ISPs in contiguous blocks, with geography being a consideration.

Key Concept

Hierarchical addressing allows for efficient allocation of addresses and reduced number of routing table entries.


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