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DHCP (Dynamic Host Configuration Protocol)

Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to dynamically assign an IP address and other network configuration parameters to each device on a network so they can communicate.

Without DHCP, a network administrator would have to manually configure the IP address, subnet mask, gateway, and DNS servers on every single device.

The DORA Process​

When a new device connects to a network (like turning on your laptop's Wi-Fi), it goes through a four-step process known as DORA to get an IP address.

  1. Discover: The client device broadcasts a DHCPDISCOVER message to the network, essentially shouting, "Are there any DHCP servers here? I need an IP!"
  2. Offer: Any listening DHCP server responds with a DHCPOFFER message, offering an available IP address.
  3. Request: The client replies with a DHCPREQUEST message, accepting the offered IP address.
  4. Acknowledge: The server sends a DHCPACK (Acknowledgment) confirming the IP has been reserved for that client, along with lease time and network settings.

What Does DHCP Provide?​

A DHCP server doesn't just hand out IP addresses. It provides a complete network configuration:

  • IP Address: The unique address for the client.
  • Subnet Mask: Defines the network boundaries.
  • Default Gateway: The IP address of the router, telling the client where to send traffic bound for the Internet.
  • DNS Servers: The IP addresses of the DNS servers so the client can resolve domain names.

DHCP Leases​

IP addresses are not assigned permanently; they are "leased."

  • The lease time defines how long the client can use the IP address (e.g., 24 hours).
  • When the lease is halfway expired, the client automatically attempts to renew it with the server.
  • If a client leaves the network and the lease expires, the server puts the IP back into the pool to be assigned to someone else.

Static IPs vs. DHCP​

For standard user devices (phones, laptops), DHCP is ideal. However, for servers (web servers, database servers, printers), you generally want a Static IP. If a server's IP address changes, clients won't know how to reach it.

You can configure static IPs by:

  1. Manually setting it in the server's OS (e.g., via Linux netplan or /etc/network/interfaces).
  2. Creating a DHCP Reservation on the router, which tells the DHCP server to always assign the same specific IP to the server's MAC address.

DHCP in Linux​

On most Linux desktop distributions, a DHCP client like dhclient or NetworkManager runs in the background.

To manually force your Linux machine to release and renew its DHCP lease:

sudo dhclient -r  # Release
sudo dhclient     # Renew