4.2DHCP, DNS, and NTP

Manually configuring an IP address on every device is impractical, so in practice we distribute them automatically via DHCP. We can reach hosts by name because DNS resolves hostnames to IPs behind the scenes, and we can correlate log timestamps because NTP synchronizes device clocks. CCNA tests whether you can, for these three "under the hood" services, judge configuration and fault isolation grounded in how they work. DHCP relay in particular is a frequent real-world trouble spot.

4.2.1DHCP DORA operation and the relay agent

• DHCP leases an address between client and server via the four DORA messages (Discover -> Offer -> Request -> Ack). The initial Discover is sent as a broadcast because the destination is not yet known (the server uses UDP 67, the client UDP 68).
• Because a router does not forward broadcasts to other segments, if the DHCP server is on a different subnet from the client, the Discover never reaches it. Bridging this is the job of the DHCP relay agent: configuring ip helper-address <server-IP> on the client-facing interface makes the router convert the received DHCP broadcast into a unicast destined for the server and forward it.
• To make a Cisco router itself a DHCP server, define ip dhcp pool <name> -> network / default-router / dns-server, and exclude addresses you do not want to hand out (such as the router's IP) with ip dhcp excluded-address.