DHCP (Dynamic Host Configuration Protocol)

DHCP (Dynamic Host Configuration Protocol) is a client-server protocol in which a device broadcasts a request and a DHCP server replies with a leased IP address plus settings such as the gateway and DNS resolvers.

Why it matters

Without DHCP every device would need manual configuration, and duplicate addresses would be common. The lease has a duration; the client renews it halfway through. Servers can reserve a fixed address for a given MAC address so important devices keep the same IP. A broken DHCP server often shows up as devices with 169.254.x.x self-assigned addresses.

Example

The exchange is known as DORA:

Discover  client broadcasts: anyone have an address?
Offer     server proposes 192.168.1.50
Request   client asks for 192.168.1.50
Ack       server confirms, lease 24h

The client now uses 192.168.1.50 until the lease expires or is renewed.

Common mistakes

• Running two DHCP servers on one network, for instance an extra router, which hands out conflicting settings.
• Assigning a static IP inside the DHCP pool, which can collide with a leased address.
• Setting lease times so long that a small address pool fills with stale devices.

Related

• IP address
• MAC address
• Default gateway
• Broadcast

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