Domain Name System (DNS)

The Domain Name System (DNS) is the internet's name-to-address directory: it translates human-friendly domain names such as example.com into the IP addresses that routers and hosts actually use. Without DNS, users would have to memorize numeric addresses for every service; with it, names stay stable even when the underlying addresses change.

DNS is a hierarchical, distributed database. A resolution starts at the root servers, which delegate to top-level domain (TLD) servers such as .com, which delegate to the authoritative name servers for a specific domain. In practice, a client sends a recursive query to its configured DNS resolver, which walks that hierarchy iteratively and caches the answer for the record's TTL, so repeat lookups are fast. Common record types include A (IPv4 address), AAAA (IPv6 address), CNAME (alias), MX (mail exchanger), NS (name server), and PTR (reverse lookup).

DNS traffic traditionally uses UDP port 53 for queries, falling back to TCP port 53 for large responses and zone transfers. On network devices, DNS matters operationally: a Cisco router or switch can be configured with name servers (ip name-server) so administrators can use hostnames in commands and troubleshooting tools like ping and traceroute, and hosts typically receive their DNS server addresses automatically via DHCP.

The CCNA exam tests DNS as a core IP network service: know what DNS does, the query flow from client to resolver to authoritative server, common record types, port 53 usage, and the router configuration commands that enable name resolution — plus how DNS interacts with services like DHCP that hand out resolver addresses.