Network protocol - DHCP working principle and DHCP Relay

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📄 Technical Background

As networks continue to grow in complexity, the number of computers often exceeds the number of IP addresses available for assignment. The widespread use of portable devices and wireless networks also means that the computer’s location often changes, and the corresponding IP address must be updated frequently. Therefore, a technology is needed to dynamically and reasonably assign IP addresses to hosts.

🌐 DHCP

Dynamic Host Configure Protocol

DHCP on Windows PC

Dynamically allocates TCP/IP information (IP addresses, masks, default gateways, DNS servers, etc.) to a large number of hosts.
Centralized management.
The information assigned is leased.

Typical DHCP Networking

role	Value
DHCP Client	A device that obtains network parameters such as an IP address through DHCP
DHCP Server	A device that assigns network parameters to DHCP clients
DHCP Relay	This parameter is optional. Forwards DHCP packets between the server and clients

DHCP workflow: Obtain IP addresses normally

DHCP message

The DHCP lease was updated. Procedure

T1: When the lease reaches 50%, the DHCP client automatically sends a DHCP REQUEST packet to the DHCP server in unicast mode to REQUEST renewal of the IP address lease. T2: When the lease reaches 87.5%, the DHCP client automatically sends a DHCP REQUEST packet to the DHCP server in broadcast mode to REQUEST renewal of the IP address lease. T3: When the lease reaches 100% and no response is received from the server, the client stops using this IP address and sends a DHCP DISCOVER message to request a new IP address. RELEASE: If the DHCP client no longer uses the assigned IP address, it can proactively send a DHCP RELEASE message to the DHCP server to RELEASE the IP address. failure DHCP address Pool: a set of assigned IP addresses