Name → IP via a hierarchical distributed database. Client asks recursively; the resolver iterates: root → TLD → authoritative, then caches the answer per TTL. UDP/53 for queries, TCP/53 for zone transfers, DNSSEC and truncated answers.

Record	Purpose
A / AAAA	IPv4 / IPv6
CNAME	alias
MX	mail exchangers
NS	authoritative servers
PTR	reverse (IP→name)
TXT	SPF/DKIM/verification
SRV	service discovery (host:port)

TTL trade-off: low = fast failover, more load; high = the opposite. Security: cache poisoning countermeasures (DNSSEC signing, source-port randomisation), don’t run open resolvers (amplification), watch for DNS tunneling.

Operations vocabulary: split-DNS, conditional forwarding, anycast resolvers, views. On a Cisco router: ip name-server 8.8.8.8 1.1.1.1, ip dns server turns it into a forwarder for labs.