Dynamic Host Configuration Protocol (DHCP) Explained

By Stephen Bucaro

In the early days of TCP/IP each device on a network had stored on its hard disk a configuration file that an administrator used to manually assign a static IP address. Later IP addresses and their associated MAC addresses where manually assigned in a central configuration file on a server, and a service called Bootstrap Protocol (BOOTP) automatically assigned the IP addresses to each device.

Today a network administrator is no longer required to maintain an IP and MAC address configuration file a server. Every device on a network is automatically assigned a unique IP address by Dynamic Host Configuration Protocol (DHCP). The DHCP service operates in the Application layer of the OSI model.

To Configure the DHCP service, a network administrator specifies a range of addresses that can be leased, and a list of addresses to be excluded (if any). A network administrator can configure the duration of the lease to be as short as a few minutes or long as forever.

With DHCP a device borrows or leases an IP address to use while it's attached to the network. The DHCP service assigns a client an IP address as it logs onto a network. After the lease duration expires, the client device will no longer be able to connect to the network. A network administrator can force a lease termination at the server or a users can force a lease termination at the client.

Leases that expire must be renegotiated in order for the client to remain connected to the network. In order to assure a proper IP address, halfway through the lease, the client will contact the DHCP server to request for a renewal of its IP address lease.

The client and server take the following steps to negotiate an IP address lease.

1. When the client is powered on, its NIC (Network Interface Circuit) detects a network connection so, via the UDP protocol it broadcasts a DHCP discover packet seeking an IP address from a DHCP server.

2. Every DHCP server on the same subnet as the client receives the request and reserves an IP address from its pool of available addresses for the client. They transmit a broadcast message containing the IP address, subnet mask, lease duration, and the IP address of the DHCP server. Because the client does not yet have an IP address, a DHCP server cannot send the information directly to the client.

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