CCNA DHCP and DNS

CCNA 200-301 exam notes · Updated 2026-10-06

DHCP and DNS are the two services every network quietly depends on, and the CCNA exam expects you to know the DORA exchange cold. This CCNA DHCP and DNS guide covers the DORA process and DHCP configuration, plus DNS on Cisco devices. When you've read it, reinforce it with a free CCNA mock test.

DHCP: DORA and Configuration

DHCP automates IP addressing through the four-step DORA exchange: Discover (client broadcasts "any DHCP server out there?"), Offer (server proposes an address), Request (client asks for it), ACK (server confirms). Discover and Request are broadcasts from the client; Offer and ACK are normally unicast. Because the first message is a broadcast, a DHCP server only hears clients on its own subnet — across subnets you need a relay agent.

Configure a Cisco router as the server:
ip dhcp excluded-address 192.168.1.1 192.168.1.10
ip dhcp pool LAN
  network 192.168.1.0 255.255.255.0
  default-router 192.168.1.1
  dns-server 8.8.8.8
  lease 7
Always exclude statically-assigned addresses first — the exam loves a pool that hands out the gateway's address. Verify leases with show ip dhcp binding and conflicts with show ip dhcp conflict.

For clients on remote subnets, put ip helper-address 10.0.0.5 on the client-facing interface — it converts the broadcast into a unicast to the DHCP server. Note the helper also forwards other UDP broadcasts (TFTP, NetBIOS) by default, which you can trim with no ip forward-protocol udp. If clients get no address, check: is the server reachable, is the pool's network correct, and is the helper on the right interface pointing at the right server?

Key exam points

DORA: Discover (broadcast) → Offer → Request → ACK. First message is a broadcast.
Exclude static addresses before defining the pool: ip dhcp excluded-address.
Pool needs network, default-router, dns-server. Verify with show ip dhcp binding.
Cross-subnet DHCP needs ip helper-address on the client-facing interface.

DNS on Cisco Devices

DNS translates names to IP addresses so humans don't have to memorise numbers. On Cisco gear, ip name-server 8.8.8.8 points the device at a resolver and ip domain-lookup enables lookups (it's on by default; no ip domain-lookup stops the annoying "translating…domain server" pause after every typo). Set ip domain-name example.com so short hostnames complete automatically.

The exam connects DNS to other topics: DHCP hands clients the DNS server (dns-server in the pool), and many features need working DNS to function (NTP pool names, CAPWAP controller discovery). Know the port — UDP 53 for queries, TCP 53 for zone transfers — because ACL questions use it.

For testing, ping and traceroute accept hostnames once DNS works. If name resolution fails, check the name-server address, confirm ip domain-lookup isn't disabled, and make sure no ACL blocks UDP 53. A device can also act as a DNS server for its clients, but that's beyond CCNA scope — just know the client-side commands.

Key exam points

ip name-server <ip> sets the resolver; ip domain-lookup enables lookups (disable to stop typo delays).
DHCP option dns-server pushes the resolver to clients.
DNS = UDP 53 (TCP 53 for zone transfers) — know it for ACL questions.
ping/traceroute accept hostnames; resolution failure = check name-server, domain-lookup, ACLs.

Related CCNA study guides

Frequently asked questions

What are the key CCNA exam points for DHCP?

For the CCNA 200-301 exam, remember: DORA: Discover (broadcast) → Offer → Request → ACK. First message is a broadcast. Exclude static addresses before defining the pool: ip dhcp excluded-address. Pool needs network, default-router, dns-server. Verify with show ip dhcp binding. Cross-subnet DHCP needs ip helper-address on the client-facing interface.

What are the key CCNA exam points for DNS on Cisco Devices?

For the CCNA 200-301 exam, remember: ip name-server <ip> sets the resolver; ip domain-lookup enables lookups (disable to stop typo delays). DHCP option dns-server pushes the resolver to clients. DNS = UDP 53 (TCP 53 for zone transfers) — know it for ACL questions. ping/traceroute accept hostnames; resolution failure = check name-server, domain-lookup, ACLs.

Which key detail about DHCP should you memorise for the CCNA 200-301 exam?

Exclude static addresses before defining the pool: ip dhcp excluded-address.

Which key detail about DNS on Cisco Devices should you memorise for the CCNA 200-301 exam?

DHCP option dns-server pushes the resolver to clients.

What is a common CCNA exam trap involving DHCP?

Pool needs network, default-router, dns-server. Verify with show ip dhcp binding.