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$ ipconfig /all
DNS Servers . . . . . . . . . . . . : a.b.c.d
                                      a.b.c.e

Why do I have two DNS servers listed? Is it something like try second in case first fails?

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up vote 10 down vote accepted

Yep, exactly correct, redundancy.

Most big DNS providers always give two addresses in the event that one fails.

Google - Configure your network settings to use the IP addresses 8.8.8.8 and 8.8.4.4 as your DNS servers or

OpenDNS - 208.67.222.222 and 208.67.220.220

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1  
+1 - In Windows the terminology is "Primary DNS" and "Backup DNS" – JNK Sep 10 '10 at 14:37
2  
I like the public verizon DNS too, 4.2.2.1 and 4.2.2.2 – JNK Sep 10 '10 at 14:37
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@JNK +1, also as a side note, on Windows, if you click the advanced button then the DNS tab, you can have a lot more than two! (I am sure you will be able to do this in other operating systems as well!) – William Hilsum Sep 10 '10 at 14:38
    
@JNK: 4.2.2.[1-5], and isn't it owned by Level3? – grawity Sep 10 '10 at 19:43
    
@grawity - it may be now, when I used it before it was a verizon public. – JNK Sep 10 '10 at 19:58

Google has a program that they have released that will run speed tests on all of the most popular DNS servers including your ISP's.

http://code.google.com/p/namebench/

This maybe be of some use to you if you wish to tweak your DNS settings.

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