Cisco Umbrella is a free and public security-focused DNS resolver provided by Cisco. Umbrella blocks malicious domains at the DNS level
These resolvers are located at 208.67.222.222, 208.67.220.220, 2620:119:35::35, and 2620:119:53::53.
https://docs.umbrella.com/deployment-umbrella/docs/point-your-dns-to-cisco