Dynamic Host Configuration Protocol (DHCP)

DHCP is the protocol used to assign IP addresses within a given network. Unlike MAC addresses, IPs are not hard-coded or unique to a device, and can be reassigned to different devices over time. This gives the network much needed flexibility, but also requires a way to manage the assignment of IPs. This is where DHCP comes in.

DHCP sits on top of the UDP protocol, and uses port 67 for the server, and port 68 for the client.

DHCP Server

The DHCP Server is a device that assigns IP addresses to devices on the network. It is typically a router or a server, and is responsible for managing a pool of IP addresses. When a device connects to the network, it broadcasts a DHCP Discover message to the network (sending to addr of all 1s). The DHCP server listens for these messages, and responds with a DHCP Offer message. This message contains an IP address that the server is willing to assign to the device. The device then sends a DHCP Request message to the server, and the server responds with a DHCP Ack message, confirming the assignment of the IP address.

The protocol also supports replicated DHCP servers, which can be used to provide fault tolerance. If a DHCP server fails, the client can simply request an IP from another server.

DHCP Client

DHCP clients are pretty ubiquitous, and are built into most devices that connect to a network. To renew an IP address, the client sends a DHCP Request message to the server, and the server responds with a DHCP Ack message, confirming the renewal.

DHCP Relay

Not all networks have a running DHCP server. In this case, a DHCP Relay can be used to forward DHCP messages to a server on another network. The relay listens for DHCP Discover messages, and forwards them to the server. The server then responds with a DHCP Offer message, and the relay forwards it to the client.