DNS

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DNS Servers Open to Public Query

IP Address Provider Location
8.8.8.8 Google
8.8.4.4 Google
156.154.70.1 DNSadvantage
156.154.71.1 DNSadvantage
208.67.220.220 OpenDNS Global, mainly US (routed via Anycast)
208.67.220.222 OpenDNS Global, mainly US (routed via Anycast)
4.2.2.1 Verizon
4.2.2.2 Verizon
194.72.9.34 BT UK
194.72.9.34 BT UK

Sources: http://theos.in/windows-xp/free-fast-public-dns-server-list/

NSLookup

NSLookup is a Name Server Lookup/Query tool which exists in both Unix and Windows distributions, and generally speaking, functionaly the same regardless of which version you're using. To start, run nslookup from the command line.

Command Description
server Displays the DNS server(s) currently being used by nslookup
server 8.8.4.4 Changes the DNS server being used/queried to 8.8.4.4
host.domain.com Attempts to resolve host.domain.com against current DNS server
109.123.76.85 Attempts a reverse lookup for IP 109.123.76.85 against current DNS server
set q=mx Set query type to MX (subsequent queries will look for Message Exchange (email) records)
set q=all Set query type back to all records
exit Quit nslookup

For example, to find the email exchange servers for a particular domain...

> set q=mx
> microsoft.com
Server:  [8.8.4.4]
Address:  8.8.4.4

Non-authoritative answer:
microsoft.com   MX preference = 10, mail exchanger = mail.messaging.microsoft.com
>