 - Reverse DNS (rDNS) is a method of resolving an IP address into a domain name, just as the domain name system (DNS) resolves domain names into associated IP addresses. One of the applications of reverse DNS is as a spam filter. Here's how it works: Typically, a spammer uses an invalid IP address, one that doesn't match the domain name. A reverse DNS lookup program inputs IP addresses of incoming messages to a DNS database. If no valid name is found to match the IP address, the server blocks that message. Although reverse DNS is fairly effective for filtering spam, it also sometimes blocks valid e-mail, at least in the current technological environment. A number of problems, including network delays and improperly configured networks or servers, can prevent legitimate messages from getting through the filter. In January 2003, AT&T WorldNet started using reverse DNS in conjunction with other anti-spam software. The company was forced to remove the filter just 24 hours after it was deployed, after subscribers reported that messages were going undelivered.
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Learn more about IP Networking |
| How to subnet: Subnetting calculations and shortcuts: Learn how subnetting works in this tip and quiz on how to calculate a subnet mask. IP address classes and bit values of octets are explained in detail. |
| IP addressing and subnetting explained: Our IP addressing and subnetting crash course provides an overview of IP addressing-related topics, including IPv4 and IPv6, subnetting, DHCP, and calculating subnet masks. |
| Troubleshooting IP Routing -- 'CCNA Official Exam Certification Library, 3rd Edition,' Chapter 7: Learn how to troubleshoot IP routing in preparation for the Cisco CCNA exam in this chapter from 'CCNA Official Exam Certification Library, 3rd Edition.' |
| IP address management -- from 'Network troubleshooting and diagnostics': Learn how IP address management and maintenance tools can help manage the scope of IP addresses on your network. |
| IP Accounting -- from Network Management: Accounting and Performance Strategies: IP Accounting, Chapter 6 of Network Management: Accounting and Performance Strategies, describes the IP Accounting features and functions in Cisco IOS, and SNMP MIB. |
| Basic IP Connectivity and CEF Troubleshooting -- Chapter 4 of "Cisco Express Forwarding": This chapter presents the general troubleshooting used on Cisco IOS routers and switches as a first step in troubleshooting IP connectivity problems. |
| LAST UPDATED: |
18 Apr 2007
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