DYNAMIC HOST CONFIGURATION PROTOCOL

A device needs an IP address to communicate on the network. One way to assign an IP address to the device is to configure it statically but if you are deploying a large infrastructure, configuring each device manually becomes impractical. Dynamic host configuration protocol (DHCP) comes to rescue.

When a client boots up for the first time (or try to join a new network), it needs to obtain an IP address to communicate. So it first transmits a DHCPDISCOVER message on its local subnet. Because the client has no way of knowing the subnet to which it belongs, the DHCPDISCOVER is an all-subnets broadcast (destination IP address of 255.255.255.255, which is a layer 3 broadcast address) and a destination MAC address of FF-FF-FF-FF-FF-FF (which is a layer 2 broadcast address). The client does not have a configured IP address, so the source IP address of 0.0.0.0 is used. The purpose of DHCPDISCOVER message is to try to find out a DHCP Server (a server that can assign IP addresses).

After receiving the discover message, the DHCP Server will dynamically pick up an unassigned IP address from its IP pool and broadcast a DHCPOFFER message to the client(*). DHCPOFFER message could contain other information such as subnet mask, default gateway, IP address lease time, and domain name server (DNS).

If the client accepts the offer, it then broadcasts a DHCPREQUEST message saying it will take this IP address. It is called request message because the client might deny the offer by requesting another IP address. Notice that DHCPREQUEST message is still a broadcast message because the DHCP client has still not received an acknowledged IP. Also, a DHCP Client can receive DHCPOFFER messages from other DHCP Servers so sending broadcast DHCPREQUEST message is also a way to inform other offers have been rejected.

When the DHCP server receives the DHCPREQUEST message from the client, the DHCP Server accepts the request by sending the client a unicast DHCPACKNOWLEDGEMENT message (DHCPACK)

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