Dynamic Host Configuration Protocol

Protocol Introduction

General

The Dynamic Host Configuration Protocol (DHCP) provides configuration parameters to Internet hosts in a client-server model. DHCP server hosts allocate network addresses and deliver configuration parameters to other (client) hosts.

DHCP consists of two components: a protocol for delivering host-specific configuration parameters from a server to a host and a mechanism for allocation of network addresses to hosts.

IP Address Allocation

DHCP supports three mechanisms for IP address allocation.

• Automatic allocation -- in which a permanent IP address is assigned to the client.
• Dynamic allocation -- in which the address is assigned for a limited period of time (a "lease").
• Manual allocation -- in which the address is assigned manually by the network administrator.

Configuration Parameters Delivery

The client sends a message to request configuration parameters and the server responds with a message carrying the desired parameters back to the client.

BOOTP Compatibility

The format of DHCP messages is based on the format of BOOTP messages due to the following reasons:

1. From the client's point of view, DHCP is an extension of the BOOTP mechanism. This behavior allows existing BOOTP clients to interoperate with DHCP servers without requiring any change to the clients' initialization software.
2. DHCP supports the BOOTP relay agent behavior.

Use of Relay Agents

DHCP does not require a server on each subnet. To allow for scale and economy, DHCP can work across routers or through the intervention of BOOTP relay agents. A realy agent listens to DHCP messages and forwards them on (and onto other network segments). This eliminates the necessity of having a DHCP server on each physical network.

Quiz

1. The two basic mechanisms in DHCP are:
   • IP adrress allocation and configuration parameters delivery.
   • Automatic and manual address allocation.
   • DHCP protocol and DHCP extentions.
   • IP address allocation by DHCP and its co-existense with other address configuration management schemes.
2. A relay agent is:
   • A router using the DHCP protocol.
   • A DHCP host you can rely on.
   • A host or router that passes on DHCP messages.
   • All of the above.
3. Relay agents where created for:
   • Saving the need for a DHCP/BOOTP server on each subnet.
   • BOOTP compatibility.
   • Interoperation of BOOTP clients with DHCP servers.
   • None of the above.

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