DHCP Fundamentals (Dynamic Host Configuration Protocol)
DHCP automatically assigns IP addresses and network configuration (subnet mask, gateway, DNS servers) to devices when they join a network - so users don't have to set this up manually.
The DHCP Process (DORA)
1. DISCOVER - Device broadcasts: "Is there a DHCP server?"
2. OFFER - DHCP server replies: "Here's an available IP"
3. REQUEST - Device says: "I'll take that IP"
4. ACK - Server confirms: "Confirmed, it's yours for X hours"
What DHCP Provides
- IP address (e.g., 192.168.1.105)
- Subnet mask (e.g., 255.255.255.0)
- Default gateway (router address)
- DNS server addresses
- Lease time (how long the device can keep this IP)
Why IT Support Needs This
If a device shows an IP like 169.254.x.x (APIPA), it means DHCP failed - the device couldn't reach a DHCP server and assigned itself a fallback address. This is a common "no internet" symptom.
Useful Commands
- Windows:
ipconfig /releasethenipconfig /renew - macOS: System Settings → Network → Renew DHCP Lease
- Linux:
sudo dhclient -rthensudo dhclient
Common Interview Talking Point
"If a laptop shows a 169.254.x.x address, I know that's an APIPA address indicating DHCP failed - I'd check the network cable/WiFi connection, then try releasing and renewing the IP lease."