DHCP is a standardized protocol defined in RFC 2131, Dynamic Host Configuration Protocol. DHCP has two main purposes: to provide TCP/IP and link-layer configuration parameters and to provide network addresses to dynamically configured hosts on a TCP/IP network.
DHCP uses a client-server model of communication. This model consists of three roles that the device can fulfill: DHCP client, DHCP server, and DHCP relay agent.
A device acting as a DHCP client (host) can request an IP address and other configuration settings from a DHCP server. Users on client devices save configuration time and effort, and need not know the network’s addressing plan or other resources and options they are inheriting from the DHCP server. A device acting as a DHCP server can service clients. By using any of three DHCP Addressing mechanisms, the network administrator saves configuration time and has the benefit of reusing a limited number of IP addresses when a client no longer needs network connectivity. The server can deliver IP addressing and many DHCP options to many clients. A device acting as a DHCP relay agent transmits DHCP messages between DHCP clients and servers.
DHCP uses User Datagram Protocol (UDP), RFC 768, as its transport protocol. DHCP messages that a client sends to a server are sent to well-known port 67 (UDP—Bootstrap Protocol and DHCP). DHCP Messages that a server sends to a client are sent to port 68.
An interface on a Palo Alto Networks firewall can perform the role of a DHCP server, client, or relay agent. The interface of a DHCP server or relay agent must be a Layer 3 Ethernet, Aggregated Ethernet, or Layer 3 VLAN interface. You configure the firewall interfaces with the appropriate settings for any combination of roles. The behavior of each role is summarized in Firewall as a DHCP Server and Client.
The firewall supports DHCPv4 Server and DHCPv6 Relay. However, a single interface cannot support both DHCPv4 Server and DHCPv6 Relay.
The Palo Alto Networks implementations of DHCP server and DHCP client support IPv4 addresses only. Its DHCP relay implementation supports IPv4 and IPv6. DHCP client is not supported in High Availability active/active mode.

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