Dynamic Host Configuration Protocol (DHCP)DHCP is an application layer protocol that dynamically assigns IP addresses and other configuration parameters to network end-stations(hosts) based on configuration policies determined by network administrators.DHCP relieves network administrators of manually configuring hosts, which can be a tedious and error-prone process when hosts oftenjoin, leave, and change locations on the network and it reclaims IP addresses that are no longer in use to prevent address exhaustion.DHCP is based on a client-server model. A host discovers the DHCP server and requests an IP address, and the server either leases orpermanently assigns one. There are three types of devices that are involved in DHCP negotiation:DHCP Server This is a network device offering configuration parameters to the client.DHCP Client This is a network device requesting configuration parameters from the server.Relay Agent This is an intermediary network device that passes DHCP messages between the client and server when theserver is not on the same subnet as the host.Topics:• DHCP Packet Format and Options• Assign an IP Address using DHCP• Implementation Information• Configure the System to be a DHCP Server• Configure the System to be a Relay Agent• Configure the System to be a DHCP Client• Configure the System for User Port Stacking (Option 230)• Configure Secure DHCP• Source Address ValidationDHCP Packet Format and OptionsDHCP uses the user datagram protocol (UDP) as its transport protocol.The server listens on port 67 and transmits to port 68; the client listens on port 68 and transmits to port 67. The configuration parametersare carried as options in the DHCP packet in Type, Length, Value (TLV) format; many options are specified in RFC 2132. To limit the numberof parameters that servers must provide, hosts specify the parameters that they require, and the server sends only those parameters.Some common options are shown in the following illustration.Figure 32. DHCP packet Format13Dynamic Host Configuration Protocol (DHCP) 293
