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 April 2014)
|Provider||Primary DNS Server||Secondary DNS Server|
|Comodo Secure DNS||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.
 OpenDNS also offers DNS servers that block adult content, called OpenDNS FamilyShield. Those DNS servers are 188.8.131.52 and 184.108.40.206. 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, and is called Policy 1. Use Policy 2 (220.127.116.11 and 18.104.22.168) to block those sites plus those with pornographic content. Use Policy 3 (22.214.171.124 and 126.96.36.199) to block all previously mentioned site categories plus those Norton deems "non-family friendly." Be sure to check out the list of things blocked in Policy 3 - there are several controversial topics in there that you may find perfectly acceptable.
 GreenTeamDNS "blocks 18 categories which include malware, botnets, dangerous websites, adult related content, aggressive/violent sites as well as advertisements and drug-related websites" according to their FAQ page. Premium accounts have more control.
 Register here with SafeDNS for content filtering options in several areas.
 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, better yet, 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.