Apparently for some routers, the mac platform does not receive the DNS information forwarded by the router. This really has to do with your router and its compatibility rather than os x. Usually most sites will work, but you will find delays in loading while some site may not load at all because the name address is not being converted to an ip.
To fix this go to your router settings.
(If u don't know how to do this its time to go look up that manual that was conveniently tossed aside
)
In those settings you should come to a summary page that lists all the info the router is getting from your isp. Note down the DNS info provided by your isp, alternatively you could just call and ask them.
Then go to your mac
Click the apple icon
Go to preferences
Click on network
Highlight what network you want to change (in my case wireless)
Select Advanced
Select DNS
Click the + sign, add the DNS you noted down.
Step back and select apply.
Should work right a way, restart if it doesn't
That is what I did and everything was working fine.
--> Another alternative is to use OpenDNS. Apparently people have had good results with it. MrGoogle the forms for it there are a lot of topics about the matter