DHCP stands for Dynamic Host Configuration Protocol. This protocol is used by DHCP clients to automatically obtain IP addresses. Most ISPs use it to make your life easy and be ready to browse the Internet right after you properly connect your network device.

Four steps, invisible to the user, take place before a host can obtain its IP address.

IP Discovery

In this stage the client probes the physical subnet to discover available DHCP servers by sending broadcast messages.

IP Lease Offer

A DHCP server receives an IP lease request from the client, it reserves an IP address and extends an IP lease offer by sending a DHCPOFFER message to the client. This message carries the client's MAC address, the IP that the server is suggesting, the subnet mask, the lease duration, and the IP address of the DHCP server carrying out the request.

IP Request

The client receives IP offers by one or more DHCP servers, but it will select only one of them and send out DHCP request message. Other servers will be informed that the client has already made a request to another DHCP and return their reserved IPs back to the pool of available addresses for other clients to take advantage of.

IP Lease Acknowledgment

In its final stage, after receiving the request from the client the DHCP server sends DHCPACK packet to the client. This packet contains configuration information that the client has requested. With this done the IP configuration process is complete.