5.3. DHCP and DNS
đź’ˇ First Principle: DHCP automates IP configuration so administrators don't manually configure every device. Clients broadcast a discovery message, and the DHCP server responds with an IP address, subnet mask, default gateway, and DNS servers. This is the DORA process: Discover, Offer, Request, Acknowledge.
DHCP and DNS are foundational services that most users never think about—until they break. When a user says "the internet is down," what do they usually mean? Often, DNS isn't working—they can reach IP addresses but names don't resolve. When devices get 169.254.x.x addresses, DHCP has failed. These two services make networks usable for humans.
DHCP solves the configuration problem: Imagine manually configuring IP addresses on 500 laptops, then tracking which addresses are assigned, and updating settings when the DNS server changes. DHCP does all of this automatically—devices request configuration, get it, and renew it periodically.
DNS solves the naming problem: Humans remember "google.com," not "142.250.80.46." DNS translates names to IP addresses—and it does this billions of times per day across the internet. Without DNS, users would need to memorize IP addresses.
What happens when they fail:
- DHCP failure: Devices get APIPA addresses (169.254.x.x) and can't communicate beyond the local link
- DNS failure: Users can ping 8.8.8.8 but "google.com" doesn't resolve—they think "the internet is down"