Lesson 3

Configuring and Managing the DHCP Server Role

DHCP is a simple, standard protocol that makes TCP/IP network configuration much easier for the administrator by dynamically assigning IP addresses and providing additional configuration information to DHCP clients automatically. It is based heavily on BOOTP but rather than pushing preconfigured parameters to the expected clients, DHCP can dynamically allocate and reclaim IP addresses from a pool of IP addresses. DHCP is an open, industry-standard protocol reducing the complexity of administering networks based on TCP/IP. It is defined by IEFT (Internet Engineering Task Force) in RFC (Request for Comments) 2131 and 2132. DHCP functions at the application layer of the OSI (Open System Interconnection) model, as defined by ISO (International Organizations for Standardization) and the ITU-T (International Telecommunication union ) Telecommunications Standards Section.

Four Key Benefits of DHCP
• Centralized administration of IP configuration
• Dynamic host configuration
• Seamless IP host configuration
• Flexibility and scalability

Additional configuration information is provided in the form of options and can be associated with reserved IPs to a vendor or user class, to a scope, or to an entire DHCP server.

APIPA is useful for providing addresses to single-segment networks that do not have a DHCP server.

DHCP Terminology

• DHCP client – Computer on the network obtaining information from the DHCP server.
• DHCP server – Computer on the network providing DHCP configuration to clients.
• DHCP lease – This defines the duration for which the DHCP lease to the client. The lease duration can be between 1 minute and 999 days. The default lease is eight days.

DHCP Message Types

• DHCPDISCOVER – Sent by the client to locate a DHCP Server.
• DHCPOFFER – Sent by the DHCP server in response to DHCPDICOVER with the offered configuration parameters.
• DHCPREQUEST – Sent by the client to signal acceptance of the offer (DHCPOFFER) from the DHCP server.
• DHCPDECLINE – Sent by the client to the DHCP Server, informing that the offer has been declined.

• DHCPACK – Sent by the DHCP server to the client to confirm.
• DHCPNACK - Sent by the DHCP server to the client to deny the DHCPREQUEST.
• DHCPRELEASE - Sent by the client to a DHCP server to relinquish an IP address and cancel the remaining lease.
• DHCPINFORM - Sent by the client to a DHCP server to ask only for additional local configuration parameters.

D.O.R.A. Discover (DHCPDISCOVER), Offer (DHCPOFFER), Request (DHCPREQUEST), Acknowledge (DHCPACK) is the process between the client and the DHCP server.
DHCP Scope The Scope determines which IP Addresses are allocated to clients. You can configure as many scopes on a DHCP server as needed for your environment.
DHCP Reservation: These are reserved IP addresses for hosts that need to have a static IP address. Examples are e-mail servers, and application servers. These are set up using the MAC address of the DHCP client computer so only the client with that address will get that reserved address.
DHCP Maintenance: Because DHCP is a key component in your organization, you must manage and monitor it. DHCP management consists of backing up and restoring the database as well as reconciling, compacting, and, in some cases, removing the database.