DHCP, DNS, and NTP Basics

Picture standing up a new office of 200 laptops. Each of these services removes a different chunk of work that would otherwise be done by hand, and each fails in a distinctive way — which is what makes them easy to tell apart when something breaks.

One design point the exam likes: servers, routers, switches, and printers are normally given static addresses, not DHCP leases. Their whole job is to be reachable at a known, unchanging address — a DNS server that moved every week would defeat its own purpose. DHCP is for the large, changing population of user devices.

A host with no address cannot have a normal conversation, so the lease exchange starts with a shout. Four messages, in a fixed order.

That reliance on broadcast has a direct consequence: DHCP broadcasts do not cross a router. If the server lives on a different subnet from the client, the Discover dies at the first router and the client never gets an offer.

! --- DHCP: reserve the gateway/servers first, then define the pool ---
R1(config)# ip dhcp excluded-address 10.10.10.1 10.10.10.10
R1(config)# ip dhcp pool LAN
R1(dhcp-config)# network 10.10.10.0 255.255.255.0
R1(dhcp-config)# default-router 10.10.10.1
R1(dhcp-config)# dns-server 10.10.20.10
R1(dhcp-config)# lease 7

! --- DNS: let the router resolve names for its own lookups ---
R1(config)# ip name-server 10.10.20.10
R1(config)# ip domain-lookup

! --- NTP: become a client of a trusted time source ---
R1(config)# ntp server 10.10.20.10

R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address       Client-ID/Hardware address   Lease expiration        Type
10.10.10.11      0100.1122.3344.55            Jul 27 2026 09:14 AM    Automatic
10.10.10.12      0100.aabb.ccdd.ee            Jul 27 2026 09:15 AM    Automatic

R1# show ntp status
Clock is synchronized, stratum 4, reference is 10.10.20.10

• ip dhcp excluded-address 10.10.10.1 10.10.10.10 reserves the gateway and the low addresses so the pool never leases the router's own IP or a statically assigned server.
• network scopes the pool; default-router and dns-server are DHCP options 3 and 6, pushed to every client.
• show ip dhcp binding proves leasing works — one row per address handed out, tied to a client MAC.
• show ntp status reporting "Clock is synchronized, stratum 4" confirms the router locked to a stratum-3 source and now sits one level below it.

• DHCP supplies address, mask, gateway, and DNS; DNS maps names to addresses; NTP agrees clocks, ranked by stratum.
• DORA = Discover, Offer, Request, Acknowledge. Discover and Request are broadcast because the client has no usable address yet.
• Router pool = ip dhcp pool + network + default-router (+ dns-server); always exclude the gateway and static servers.
• ip helper-address goes on the client-facing interface, and is needed whenever server and client are on different subnets.
• Verify with show ip dhcp binding for leases and show ntp status for "Clock is synchronized".
• Ping by address working while ping by name fails is DNS — and proves DHCP is fine.