How DHCP Works
When a DHCP client computer is started, the TCP/IP software is loaded into memory and starts to operate. However, because TCP/IP has not been given an IP address yet, it is incapable of sending or receiving directed datagrams. TCP/IP can, however, transmit and listen for broadcasts. This capability to communicate via broadcasts is the basis for how DHCP works. The process of leasing an IP address from the DHCP server involves four steps:
DHCPDISCOVER— The DHCP client initiates the process by broadcasting a datagram destined for UDP port 68 (used by BOOTP and DHCP servers). This first datagram is known as a DHCP Discover message, which is a request to any DHCP server that receives the datagram for configuration information. ...