Many enterprise routers hardcode 3.3.3.3 as a tertiary or backup DNS resolver to maintain connectivity if primary servers fail. 3.3.3.3 vs. Popular Public DNS Resolvers
In networking, IP addresses with repeating octets—such as 1.1.1.1, 8.8.8.8, and 9.9.9.9—are highly prized. They are easy for human administrators to remember and type into local configuration files. dns 3.3.3.3