Most of the time, your ISP automatically assigns a primary and secondary DNS server when your router or computer requests network information via DHCP. But what if those ISP-provided DNS servers aren't reliable or you're troubleshooting an issue and you suspect that DNS might not be working properly?
Luckily there are several free DNS servers that you can use instead of the ones assigned to you. See How Do I Change DNS Servers? for help if you're not sure what to do.
Free & Public DNS Servers (Updated May 2013)
|Provider||Primary DNS Server||Secondary DNS Server|
|Comodo Secure DNS||220.127.116.11||18.104.22.168|
|ScrubIT6 (may be out of service)||22.214.171.124||126.96.36.199|
 The DNS servers listed above as Level3 will automatically route to the nearest DNS server operated by Level3 Communications, the company that provides most of the ISPs in the US their access to the Internet backbone.
 Google also offers IPv6 public DNS servers: 2001:4860:4860::8888 and 2001:4860:4860::8844.
 Securly's DNS servers block adult content by default, including of course X-rated content but also adult oriented pages on websites like Wikipedia, adult themed searches on Google, and more. The grown-ups in the house can sign up here to bypass all restrictions.
 OpenDNS also offers DNS servers that block adult content, called OpenDNS FamilyShield. It's also free but you do have to sign up here. A premium DNS offering is also available, called OpenDNS Home VIP.
 The Norton ConnectSafe DNS servers listed above block sites hosting malware, phishing schemes, and scams. Use 188.8.131.52 and 184.108.40.206 to block those sites plus those with pornographic content. Use 220.127.116.11 and 18.104.22.168 to block all previously mentioned site categories plus those Norton deems "non-family friendly."
 ScrubIT also offers "filtered" DNS servers that restrict access to pornographic and malicious web sites. Those DNS servers are 22.214.171.124 and 126.96.36.199 and you can read more about it here.
 Register here with SafeDNS for content filtering options in several areas.
 DNSResolvers.com says they do not "alter" the traffic in any way, including correcting "typos and non-existent domains."
 The DNS servers listed here for OpenNIC are just two of many in the US and across the globe. Instead of using the OpenNIC DNS servers listed above, see their complete list of public DNS servers here and use two that are close to you or let them tell you that automatically here. OpenNIC also offers some IPv6 public DNS servers.
 These Public-Root DNS servers are the only two currently operating in the United States but if you're located ouside the US, see their complete list here and choose the best servers based on your location.
 The censurfridns.dk DNS servers are uncensored, operated by a privately funded individual, and are physically located in Denmark. You can read more about them here. IPv6 DNS servers are also available at 2002:d596:2a92:1:71:53:: and 2002:5968:c28e::53
 Hurricane Electric also has an IPv6 public DNS server available: 2001:470:20::2.
 puntCAT is physically located near Barcelona, Spain. The IPv6 version of their DNS server is 2a00:1508:0:4::9.
Note: Primary DNS servers are sometimes called preferred DNS servers and secondary DNS servers are sometimes called alternate DNS servers. Primary and secondary DNS servers can be "mixed and matched" to provide another layer of redundancy.