c# qr code generator code project How a Reverse Lookup Query Works in Visual C#

Maker QR Code in Visual C# How a Reverse Lookup Query Works

How a Reverse Lookup Query Works
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A reverse lookup query maps an IP address to a name. Troubleshooting tools, such as the nslookup command-line tool, use reverse lookup queries to report back host names. Additionally, certain applications implement security based on the capability to connect to names, not IP addresses.
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Supporting TCP/IP
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Because the DNS distributed database is indexed by name and not by IP address, a reverse lookup query would require an exhaustive search of every domain name. To solve this problem, in-addr.arpa was created. This special second-level domain follows the same hierarchical naming scheme as the rest of the domain namespace; however, it is based on IP addresses, not domain names, as follows:
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Subdomains are named after the numbers in the dotted-decimal representation of IP addresses. The order of the IP address octets is reversed. Companies administer subdomains of the in-addr.arpa domain based on their assigned IP addresses and subnet mask.
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For example, Figure 13-12 shows a dotted-decimal representation of the IP address 192.168.16.200. A company that has an assigned IP address range of 192.168.16.0 to 192.168.16.255 with a subnet mask of 255.255.255.0 has authority over the 16.168.192.in-addr.arpa domain.
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. arpa In-addr
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Figure 13-12
The in-addr.arpa domain is used in reverse lookup queries.
On the CD
At this point, you should view the multimedia presentation The Name Resolution Process, which is available in the Multimedia folder on the CD-ROM accompanying this book. This presentation will give you a deeper understanding of the name resolution process.
Lesson Review
The following questions are intended to reinforce key information presented in this lesson. If you are unable to answer a question, review the lesson materials and try the question again. You can find answers to the questions in the Questions and Answers section at the end of this chapter.
Lesson 3
Overview of Name Resolution 13-37
1. What is a forward lookup query and how is it resolved
2. In DNS name resolution, which of the following statements about TTL are correct (Choose all that apply.) a. TTL is the length of time a query can exist before it is discarded. b. Shorter TTL values help ensure that data about the domain namespace is more current across the network. c. Longer TTL values increase the amount of DNS traffic. d. Longer TTL values cause the cached values to be retained longer. 3. Which of the following statements about DNS name and address resolution are correct (Choose all that apply.) a. The DNS distributed database is indexed by both names and IP addresses. b. The top-level domain in-addr.arpa is used for both forward and reverse queries. c. In the in-addr.arpa domain, the order of the IP address octets is reversed. d. Troubleshooting tools, such as the nslookup command-line tool, use reverse lookup queries to report back host names.
Lesson Summary
A forward lookup query resolves a name to an IP address, and a reverse lookup query resolves an IP address to a name. Name servers cache query results to reduce DNS traffic on the network. The DNS distributed database is indexed by name, not by IP address, so inaddr.arpa (a special second-level domain) was created. It is based on IP addresses instead of domain names.
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Supporting TCP/IP
Lesson 4: Configuring a DNS Client
There are several methods available for configuring TCP/IP name resolution on Windows XP Professional clients. The method covered in this lesson uses DNS to provide name resolution. A second method is to use a HOSTS file. For networks without access to a DNS name server, creating a HOSTS file, which is a manually maintained local file, can provide host-to-IP address name resolution for applications and services. HOSTS files can also be used in environments in which name servers are available, but not all hosts are registered, perhaps because some hosts are only available to a limited number of clients. If there is a computer on your network that is running Windows 2000 Server or Windows Server 2003 and has the DNS Service installed and configured on it, you should use DNS for name resolution. This lesson shows you how to configure your computer as a DNS client.
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