Installing and Administering Internet Services

Chapter 6 255
Dynamic Host Configuration Protocol (DHCP)
DHCP Components and Concepts
DHCP Leases
The DHCP server has control of the IP address block. It grants DHCP
clients permission to use IP addresses on a lease basis. The IP address is
“leased” to the client for a fixed amount of time. The administrator sets
the lease time, which can last from 120 seconds to infinity.
During the lease, DHCP guarantees that the IP address assigned to the
client will not be re-assigned to another client.
Before the lease timeexpires, the DHCP client automatically requests an
extension on its lease. As long as the client can contact the DHCP server,
the server will renew the lease.
For example, when client A reboots, it attempts to renew the lease it had
before being powered off. If client A was powered off when the renewal
time elapsed, it can be assigned a different IP address. If the IP address
is still unassigned when client A comes back online, the server can
assign the same IP address to client A. But if the server assigned the IP
address to client B while client A was off-line, client A will be assigned a
different IP address.
DHCP Transactions: Basic Operation
This section covers the basic interaction between a client and DHCP
server to assign an IP address to a client on the network.
Step 1. A DHCP transaction begins when a client sends out a DHCP DISCOVER
packet. This is usually a broadcast packet. At minimum, the packet will
contain only the client’s hardware address.
Step 2. The server receives the DHCP DISCOVER packet. If an IP address on
the client subnet is available and the server is willing to grant a lease, it
makes an offer by sending a DHCP OFFER packet to the client. The offer
packet contains the:
• Proposed IP address for client
• Server’s name
• Server’s IP address
• Other configuration information
Step 3. The client receives the DHCP OFFER packet. The client might receive
more than one offer if you have more than one DHCP server on the