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An individual server that responds to Domain Name System queries that are directed to the Internet Protocol (IP) address of one of the servers that is authoritative for the root zone. For example, an “instance of the ICANN Managed Root Server" refers to a root server that answers queries sent to the IP address of the authoritative name server operated by ICANN.
Hundreds of root server instances exist around the world. When a resolver submits a query to a root server, anycast routing relays the query packet to the nearest (in terms of routing layout) root server instance. If a local instance is unavailable (perhaps due to a power outage or a network problem), routers automatically redirect the query to the next nearest instance.