DHCP: How Devices Get Their IP Addresses

Static IP assignment doesn't scale. I tried it once on a network with 50 devices - tracking which IP went where became a spreadsheet nightmare, and duplicate addresses caused random connectivity issues. DHCP automates all of this.

What DHCP Does

When a device connects to a network, DHCP automatically provides:

IP address
Subnet mask
Default gateway
DNS servers
Lease duration

No manual configuration needed. The device just works.

The DORA Process

DHCP uses a four-step handshake called DORA:

1. Discover - New device broadcasts "I need an IP address" to everyone on the network segment. It uses 0.0.0.0 as source since it doesn't have an address yet.

2. Offer - DHCP servers respond with available addresses from their pools. The offer includes all the network configuration.

3. Request - Device picks one offer (usually the first) and broadcasts "I'm taking this one." This tells other servers to release their offers.

4. Acknowledge - Selected server confirms the assignment. Device configures its interface and starts communicating.

Lease Lifecycle

DHCP addresses aren't permanent - they're leased for a specific duration.

T1 timer (50%) - At half the lease time, the device tries to renew directly with the original server. If successful, the lease resets.

T2 timer (87.5%) - If the original server doesn't respond, the device broadcasts to any available DHCP server. Failover mechanism.

Expiration - No response? The device loses its IP. You'll see it grab a 169.254.x.x address (APIPA) - that's the sign DHCP failed.

Production Considerations

Redundancy matters. I always deploy DHCP with failover. Options:

Split scope (80/20 between servers)
Hot standby
Load balancing

DHCP relay for multi-subnet networks. DHCP broadcasts don't cross routers. Configure ip helper-address on routers to forward requests to central DHCP servers.

Lease duration trade-offs:

Short leases (1-4 hours) - Good for guest networks, high device turnover
Long leases (days/weeks) - Reduces DHCP traffic, better stability during server outages

Troubleshooting

When a device gets 169.254.x.x, DHCP failed. Check:

Is the DHCP server running?
Is there a route between client and server? (relay configured?)
Is the address pool exhausted?
Are there network issues blocking UDP 67/68?

# Windows - release and renew
ipconfig /release
ipconfig /renew

# Linux - restart DHCP client
sudo dhclient -r
sudo dhclient

# Check DHCP lease info
cat /var/lib/dhcp/dhclient.leases

Key Takeaways

DORA: Discover → Offer → Request → Acknowledge
169.254.x.x addresses mean DHCP failed - investigate immediately
Lease timers T1 (50%) and T2 (87.5%) handle renewal and failover
Always run redundant DHCP in production
Multi-subnet networks need DHCP relay configured on routers