Even if your router is managing DNS settings, you can set customized server addresses for individual Macs on the Network system preference pane. To do so, launch the Network System Preferences panel.
Setup DNS Server Running OS X Server 5 - krypted Sep 21, 2015 Changing DNS info on a Macintosh - Verizon Fios Community I have just read a closed thread on changing one's DNS on an Actiontec modem/router. I have one iMac and two MacBook Pros on my LAN, all running High Sierra. I know that I can change my DNS on these computers, one by one, by going to System Preferences > Network > Advanced > DNS and inputting the primary & secondary addresses of the new DNS.
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DNS servers greyed out - Apple Community Jun 12, 2008 Configure OS X Settings for Xfinity® Internet – Xfinity Configure OS X Settings for Xfinity Internet. Learn how to use Xfinity Internet on your Mac devices. How to Use OpenDNS or Google DNS on Your Mac Sep 11, 2017