How does DHCP (Dynamic Host Configuration Protocol) work?

Dynamic Host Configuration Protocol (DHCP) is a network protocol that enables devices to obtain IP addresses and other network configuration information dynamically. DHCP simplifies the process of network administration by automatically assigning IP addresses to devices on a network without manual intervention. Below is a detailed technical explanation of how DHCP works:

  1. Discover (DHCPDISCOVER):
    • When a device (client) connects to a network and needs an IP address, it sends a DHCPDISCOVER broadcast message to the local network. This message is broadcast to all devices on the network, seeking a DHCP server.
  2. Offer (DHCPOFFER):
    • DHCP servers on the network receive the DHCPDISCOVER message and respond with a DHCPOFFER unicast message. This message contains an available IP address, subnet mask, lease duration, and other configuration parameters.
    • If multiple DHCP servers are present, the client may receive multiple offers. It will choose one based on criteria like response time or configuration options.
  3. Request (DHCPREQUEST):
    • The client, having received one or more DHCPOFFER messages, selects an offer and sends a DHCPREQUEST broadcast message to notify the chosen DHCP server of its decision.
    • The broadcast message is sent to all DHCP servers, but the servers not chosen will ignore the request.
  4. Acknowledgment (DHCPACK):
    • The DHCP server that received the DHCPREQUEST message responds with a DHCPACK message to confirm the allocation of the requested IP address to the client.
    • The DHCPACK message includes the IP address, subnet mask, gateway information, DNS servers, and any other configuration parameters.
    • If the DHCP server cannot fulfill the request, it may respond with a DHCPNAK (Negative Acknowledgment) message, and the client needs to restart the process.
  5. Lease Time and Renewal:
    • The client now has a lease on the assigned IP address, which is valid for a specific duration defined by the DHCP server. The lease time prevents IP addresses from being tied up indefinitely.
    • Before the lease expires, the client may attempt to renew the lease by sending a DHCPREQUEST message to the DHCP server that originally assigned the IP address. The server may either renew the lease or provide a new IP address.
  6. Release:
    • When a device disconnects from the network or no longer requires an IP address, it sends a DHCPRELEASE message to the DHCP server, indicating that it is releasing the assigned IP address.