Cisco Certified Network Associate (CCNA) 200-301 · Free study guide

Objective 4.3 — Explain DHCP and DNS roles

DHCP and DNS often appear together in client configuration, but they solve different problems. DHCP leases network parameters. DNS answers name questions. Troubleshoot them as separate services connected by a dependency chain.

DHCP's configuration role

A new client uses a broadcast-oriented discover, offer, request, and acknowledgment exchange to obtain a lease. The server can supply an IP address, mask or prefix, default gateway, DNS resolver, lease duration, and other options. The chosen scope must match the client's subnet, which a local server knows from its receiving interface or a remote server learns through relay context.

A valid lease does not guarantee the gateway routes or the DNS server answers. Inspect every leased parameter.

DNS name resolution

DNS maps names to records. A client sends a query to its configured resolver, which can answer from cache or pursue the namespace through other servers. Common host resolution returns address records, but DNS also stores aliases, mail routing, service, and delegation information.

An authoritative server owns answers for a zone; a recursive resolver performs queries on a client's behalf. One system can provide both roles in a small design, but the functions remain distinct.

How the services interact

DHCP commonly tells a client which DNS resolver and search domain to use. Dynamic systems may update DNS when leases are created, but that is an integration—not the same protocol. A DHCP failure can leave the client without a resolver setting; a DNS failure can break names while the DHCP lease remains valid.

Worked scenario

A workstation has a valid address, mask, and gateway and can reach 198.51.100.20, but portal.example.test fails. Inspect the configured DNS server, query it directly, and verify the returned record. Releasing the DHCP lease is not the first repair unless the leased resolver option is wrong.

Another workstation self-assigns an address and has no gateway. That points toward the DHCP path before DNS.

Verification sequence

For DHCP, inspect lease state, server scope, exclusions, relay where present, and the four-message exchange. For DNS, inspect client resolver settings, query result, record type, authoritative path, cache, and network reachability. Test an IP and a name separately to localize the dependency.

Common traps

• Saying DHCP translates names.
• Saying DNS assigns default gateways.
• Treating one successful lease as proof the scope has capacity.
• Blaming routing when only names fail.
• Forgetting a relay when the server is across a router.

Readiness checklist

• I can describe the DHCP lease flow.
• I can separate resolver and authoritative DNS roles.
• I know how DHCP supplies DNS parameters.
• I can build independent service tests.