Technical data

Configuring the DHCP Server
7.1 Key Concepts
Based on the BOOTP functionality, DHCP is built on the client/server model:
• The DHCP server is a host that provides initialization parameters.
• The DHCP client is a host that requests initialization parameters from a
DHCP server. A router cannot be a DHCP client.
7.1.1 How DHCP Operates
DHCP consists of two components:
• A mechanism for allocating network addresses to clients
• A set of rules for delivering client-specific configuration parameters from a
DHCP server to a client
DHCP operates as follows:
• When a DHCP client boots, it broadcasts a DHCP request, asking that any
DHCP server on the network provide it with an IP address and configuration
parameters.
• A DHCP server on the network authorized to configure this client sends the
client a reply that offers an IP address.
• When the client receives the offer, it can accept it or wait for other offers from
other servers on the network.
• Once the client accepts an offer, it sends an acceptance message to the server.
• When the server receives the acceptance message, it sends an
acknowledgment with the offered IP address and any other configuration
parameters that the client requested. (The server only responds to specific
client requests; it does not impose any parameters on the client.)
• If the dynamic address allocation method is used, the IP address offered to
the client has a specific lease time that determines how long the IP address is
valid.
During the lifetime of the lease, the client repeatedly asks the server to renew. If
the client chooses not to renew, the lease expires.
Once the lease expires, the IP address can be recycled and given to another client.
When the client reboots, it can be given the old address if available or assigned a
new address.
For more information about how DHCP operates, see RFC 2131 and RFC 1534.
7.1.2 How DHCP Allocates IP Addresses
With TCP/IP Services, DHCP uses the dynamic and static IP address-mapping
methods outlined in Table 7–1 to service DHCP and BOOTP-only client
requests.
7–2 Configuring the DHCP Server