Internet Routing Architectures, Second Edition
Figure 3-24. Local-Use Address Formats
Local-use addresses have local meaning. Link-local addresses have local meaning to a particular segment, and site-local addresses have local meaning to a particular site.
Companies that are not connected to the Internet can easily assign their own addresses without a need for requesting prefixes from the global address space. If the company later decides to connect to the global Internet, a REGISTRY ID, PROVIDER ID, and SUBSCRIBER ID will be assigned with the already assigned local addresses. This is a major advantage over having to replace all private addresses with global addresses or using NAT tables to get things working in the IPv4 addressing scheme.
Looking Ahead
IP addresses and addressing issues are basic elements of interdomain routing. IP addressing defines where certain information can be found, but it does not give any indication of the pathway by which the information is to be accessed. Hosts need a mechanism to exchange information about destinations and to calculate the optimal path to reach a given destination. This mechanism, of course, is routing.
This chapter concludes all the fundamental material required before proceeding to further study of Internet routing architectures. The next chapter covers the basics of interdomain routing, building on the concepts of addressing, global networks, and domains that have already been discussed. Routing protocols in general, and BGP in particular, will be discussed, with implementation details on BGP to follow in Chapter 5 and beyond.
Frequently Asked Questions
Q—
How is VLSM different from subnetting?
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A—
It is not. VLSM is an extension of basic subnetting whereby the same Class A, B, or C address is subnetted by using masks of different lengths.
Q—
Why do I need VLSM?
A—
VLSM provides a more efficient way to assign IP addresses. It provides more flexibility in assigning adequate numbers of hosts and subnets given a limited number of IP addresses.
Q—
What is the difference between CIDR and supernetting?
A—
Classless inter-domain routing is the mechanism that allows networks to advertise both supernets and subnets outside the normal bounds of a classful network number. Supernetting is a representation that allows masks that are shorter than the natural masks, hence creating supernets.
Q—
Is the classful model the cause of the growth in the global routing tables?
A—
No. The growth of the routing tables is due to more organizations connecting to the Internet. The classful model does not offer a scalable solution to cope with such growth.
Q—
I have a network that uses older protocols such as RIP-1 and IGRP. What issues should I consider in deciding whether to upgrade to newer protocols that support VLSM and CIDR?
A—
If you feel that implementing VLSM and CIDR can help you utilize your address space more efficiently and will give you better route summarization capabilities, you should upgrade. One issue could be whether your current hardware is capable of running newer protocols that might need extra processing or memory requirements. Of course, this depends on the protocol to which you are upgrading. Other issues involve the coexistence of new and old protocols. Because network upgrades are usually done in stages, you will be faced with situations where both older and newer protocols are running concurrently. Because older classful routing protocols cannot deal with VLSM or CIDR, you should not be surprised that extensive use of
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static routing might be required to ensure connectivity in your domain during the transition period.
Q—
Can I aggregate any routes in my routing table?
A—
You can aggregate only routes that you administer. Aggregating routes that are not an extension of your domain could create black holes.
Q—
If I leave my provider, can I keep my IP addresses?
A—
For the purposes of better aggregation, today's routing practices recommend (sometimes require) that you return the old addresses and get addresses from your new provider. Ask your provider for its policies.
Q—
I have some hosts that require Internet connectivity and others that do not. Can I use private addresses on some hosts and not others?
A—
Yes, you can use both private and global addresses in the same network. When advertising routes to your provider, you advertise only the legal (nonprivate) networks.
Q—
I need to connect to the Internet, and not all my addresses are registered. I can't afford to renumber. What should I do?
A—
You could use Network Address Translation (NAT) to map your invalid address to a legitimate pool of addresses that you get from your provider.
References
1.RFC 791, "Internet Protocol (IP)," http://www.isi.edu/in-notes/rfc791.txt
2.RFC 917, "Internet Subnets," http://www.isi.edu/in-notes/rfc917.txt
3.RFC 1878, "Variable Length Subnet Table for IPv4," http://www.isi.edu/innotes/rfc1878.txt
4.RFC 1519, "Classless Inter-Domain Routing (CIDR)," http://www.isi.edu/innotes/rfc1519.txt
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5.RFC 1918, "Address Allocation for Private Internets," http://www.isi.edu/innotes/rfc1918.txt
6.RFC 1541, "Dynamic Host Configuration Protocol," http://www.isi.edu/innotes/rfc1541.txt
7.RFC 1631, "The IP Network Address Translator," http://www.isi.edu/innotes/rfc1631.txt
8.RFC 1884, "IP Version 6 Addressing Architecture," http://www.isi.edu/innotes/rfc1884.txt
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Part II: Routing Protocol Basics
Although this book is primarily concerned with exterior gateway protocols—routing between different autonomous systems—it makes sense to look at internal gateway protocols as a first step because, conceptually and in practice, the two will affect each other's behavior. Thus, Chapter 4 begins with a consideration of protocols intended for routing within an autonomous system before moving into exterior gateway protocols. Chapter 4 concludes with an overview of the particular exterior gateway protocol, Border Gateway Protocol (BGP), which we will focus on. Chapter 5 contains provides an overview of how BGP Version 4 (BGP-4) operates and how and what it negotiates with neighboring routers. In addition, Chapter 5 covers the multiprotocol extensions to BGP-4, Capabilities Negotiation with BGP-4, and the TCP MD5 Signature Option for BGP. Understanding the basics of BGP, as described in Part II, is necessary before we can put the protocol's capabilities to use in practical routing design problems throughout the rest of the book.
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