DHCP Dynamic Host Configuration Part 7 NVCC Professional Development TCP/IP

DHCP Dynamic Host Configuration Protocol DHCP works by providing a process for a server to allocate IP information to clients. Clients lease the information from the server for an administratively defined period. When the lease expires, the client asks for an address again, typically is reassigned the same address. Administrators set up DHCP servers to assign addresses from predefined pools. DHCP servers can also offer other information, such as DNS server addresses, WINS server addresses, and domain names.

Major DHCP Features Automatic allocation – DHCP assigns a permanent IP address to a client. Manual allocation – The IP address for the client is assigned by the administrator. DHCP conveys the address to the client. Dynamic allocation – DHCP assigns, or leases, an IP address to the client for a limited period of time.

DHCP Dynamic Host Configuration Address Pooling DHCP enables DHCP clients on an IP network to obtain their configurations from a DHCP server. Less work is involved in managing an IP network when DHCP is used. The most significant configuration option the client receives from the server is its IP address.  Works in Client/Server mode  Host (DHCP Client) on a network obtain configurations from a Server (DHCP Server)  The Server manages the configurations by answering requests from Clients  Cannot be used for devices that need STATIC addresses i.e. routers, switches & servers.  The Server issues LEASES  Client requests use UDP port 67, server responses use UDP port 68.

Introducing DHCP Client sends a broadcast request, server receives and notes that the Client IP address field is empty. Server replies with an IP address for the Client to use.

DHCP Operation DHCP client broadcasts a Discover packet on the local subnet. DHCP servers send an Offer packet with lease information. (Servers may ping an address first to ensure that it is not already in use.) DHCP client selects lease and broadcasts a Request packet. Selected DHCP server sends ACK packet.

DHCP Relay and Helper Addresses DHCP clients broadcast their requests. Routers do not forward broadcasts. If no DHCP server is on the client segment, DHCP cannot happen. Helper addresses configured on the router allow the router to send the request to a specified DHCP server on another segment.

DHCP Relay with Helper Addresses By default, the ip helper- address command forwards the following eight UDP services: Time TACACS DNS BOOTP/DHCP Server BOOTP/DHCP Client TFTP NetBIOS Name Service NetBIOS datagram Service