How DHCP Work?

Dynamic Host Configuration Protocol (DHCP) is a network management protocol used to automatically assign IP addresses and other network configuration parameters to devices on a network. Here's a step-by-step explanation of how DHCP works:

1. DHCP Discovery (DHCPDISCOVER)

• When a device (client) connects to a network, it sends a DHCPDISCOVER message to the network. This is a broadcast message (sent to all devices on the network) because the client does not yet have an IP address or know the DHCP server's address.
• The message includes the client's MAC address and may include a request for specific configuration parameters.

2. DHCP Offer (DHCPOFFER)

• DHCP servers on the network receive the DHCPDISCOVER message and respond with a DHCPOFFER message. This message includes:
• A proposed IP address for the client.
• Subnet mask, lease duration (how long the IP address is valid), and other network settings (e.g., default gateway, DNS servers).
• The DHCPOFFER is also a broadcast message (unless the DHCP server knows the client's MAC address and can send it directly).

3. DHCP Request (DHCPREQUEST)

• The client selects one of the offered IP addresses (usually the first one it receives) and sends a DHCPREQUEST message to the DHCP server. This message:
• Confirms the acceptance of the offered IP address.
• May also request additional configuration parameters.
• The client may receive multiple DHCPOFFER messages from different DHCP servers, but it only responds to one.

4. DHCP Acknowledgment (DHCPACK)

• The DHCP server that sent the selected DHCPOFFER responds with a DHCPACK message. This message:
• Finalizes the IP address assignment.
• Includes all the agreed-upon configuration parameters (e.g., subnet mask, default gateway, DNS servers, lease duration).
• The client now configures its network interface with the assigned IP address and other settings.

5. Lease Renewal

• The IP address assigned to the client is leased for a specific period (lease duration). Before the lease expires, the client must renew it to continue using the IP address.
• Renewal Process:
1. When half of the lease time has passed, the client sends a DHCPREQUEST message directly to the DHCP server to renew the lease.
2. If the server approves, it sends a DHCPACK message with the updated lease duration.
3. If the server does not respond, the client waits until 87.5% of the lease time has passed and then broadcasts a DHCPREQUEST to any available DHCP server.

6. Lease Expiration or Release

• If the client no longer needs the IP address (e.g., it disconnects from the network), it can send a DHCPRELEASE message to the DHCP server to release the IP address.
• If the lease expires and the client cannot renew it, the IP address is returned to the DHCP server's pool of available addresses.

Key Points:

• Broadcast Communication: Initially, DHCP uses broadcast messages because the client does not have an IP address.
• Lease Management: DHCP assigns IP addresses temporarily, and clients must renew their leases periodically.
• Centralized Management: DHCP simplifies IP address management by automating the assignment process and reducing the risk of conflicts.

Example Scenario:

1. A laptop connects to a Wi-Fi network.
2. The laptop sends a DHCPDISCOVER message.
3. The DHCP server on the network responds with a DHCPOFFER.
4. The laptop sends a DHCPREQUEST to accept the offer.
5. The DHCP server sends a DHCPACK to confirm the assignment.
6. The laptop configures its network settings and connects to the internet.

This process ensures that devices can join a network and communicate without manual configuration.