The term DNS stands for

Concept

Every host on the Internet is reached by a numeric IP address, but people remember words, not numbers. A naming service therefore sits between the two: it defines a hierarchical space of human-readable labels, and it runs a distributed set of servers that translate any such label into the numeric address a packet actually needs. The service is named after exactly those three ideas — the label space, the label itself, and the machinery that resolves it.

Applying it here

Read the abbreviation letter by letter against that idea:

• D — Domain: the hierarchical label space, read right to left as root, then a top-level domain such as .com, then example.com, then www.example.com.

• N — Name: the human-readable label a person types, as distinct from the numeric address the network routes on.

• S — System: the worldwide set of cooperating servers together with the query-and-response protocol (port 53) that turns a label into an address.

How a lookup actually runs

So DNS expands to Domain Name System. A single lookup makes the three parts visible:

1. A browser is handed www.example.com and asks its local resolver for the matching address.

2. The resolver queries a root server, which does not know the address but points it to the servers for the .com top-level domain.

3. The .com servers in turn point it to the authoritative servers for example.com.

4. The authoritative server returns the address record, for example 93.184.216.34.

5. The browser opens its connection to that numeric address, and every router along the path forwards the packets on that address rather than on the label; the label's job ends once it has been resolved.

Contrasting the other expansions

The near-miss expansions each break one of the three parts:

• Dependent Name Server keeps the naming idea but names a single machine and its reliance on another host; a name server is one component inside the service, not the service as a whole.

• Downloadable New Software describes a category of distributable programs, so it supplies no label space and no resolution step.

• Defense Nuclear System belongs to military and atomic-energy vocabulary and has no place in the TCP/IP protocol suite.

DNS therefore stands for Domain Name System.