How the Domain Name System Works

This lesson addresses the following questions:

• How is the DNS hierarchy structured?

• How does caching reduce the load on the DNS infrastructure?

• How does the distributed nature of DNS improve robustness?

DNS hierarchy

DNS is not a single server. It is a vast infrastructure comprising name servers organized in a hierarchy.

A lookup commonly involves the following roles. The recursive resolver is a client of the authoritative hierarchy, not another level of that hierarchy:

1. DNS resolver: Resolves queries on the client’s behalf and can reuse cached answers. It may be operated by an ISP, an organization, or another provider.

2. Root-level name servers: Supply referrals for top-level domains such as .com, .edu, and .io.

3. Top-level domain (TLD) name servers: Supply delegations to the name servers responsible for domains below the TLD.

4. Authoritative name servers: Answer from the zone data they serve. A domain owner may operate them directly or use a provider; their records can contain addresses or other DNS data.