Click Update, which will temporarily make the network unavailable while the base station restarts with the new settings.
airport utility dns
AirPort Utility allows entering DNS server settings that are then used when network addresses are assigned to devices as they connect.
If you'd like to be able to easily swap between a public DNS server and your ISP's servers, save your set up first by choosing File > Export Configuration File. Name the file descriptively so you can identify it later, like "AirPort Main ISP DNS Servers."
Now make the changes as in the steps above, and after the base station reboots, select File > Export Configuration File, and save with a similarly descriptive name, such as "AirPort Main Google Public DNS."
When you want to swap between them, following steps 1 to 3 above, then select File > Import Configuration Server, select the file, then click Update. The base station will restart with the new configuration.
Hold me closer, tiny DNSer
There's a downside to picking a public DNS service instead of one operated by your ISP. Content distribution networks (CDNs) are heavily used by companies large and small to push images, movies, and other media "closer" to end users. They accomplish this by instead of running massive servers in a few places (or, for a small company, one server somewhere), having many thousands of servers around the globe.
These servers are located either in major network interchanges or inside the networks operated by ISPs and companies. When users request webpages or other downloads, their device, router, or ISP's DNS request returns a result for the IP address of a server that has the fewest number of network hops. Akamai is a popular CDN, while Amazon runs one as part of its on-demand services division. Apple built its own and also contracts with others.
However, public DNS servers can't pass along the origin point of a DNS request the way a broadband provider can: the CDN network sees a request from the DNS server, not from your point in the network. This can result in your system getting a connection to a CDN server that's very "far" from you in network terms. A download that should flow at 10Mbps instead pokes along at 500Kbps.
Keep that in mind if it happens to you; the solution is to switch back (at least temporarily) to an ISP's DNS servers as described above.
The next time you have heartbreak of Internet failure, DNS may not be the problem. An errant backhoe cutting through cabling or sheer incompetence may be at work. But it's worth a shot, and for some ISPs, switching to public DNS can often improve overall performance.
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