Users Guide

DHCP server Network device offering configuration parameters to the client.
DHCP client Network device requesting configuration parameters from the server.
Relay agent Intermediary network device that passes DHCP messages between the client and the server when the server is
not on the same subnet as the host.
Packet format and options
The DHCP server listens on port 67 and transmits to port 68. The DHCP client listens on port 68 and transmits to port 67.
In the DHCP packet format, configuration parameters are options in the DHCP packet in type, length, value (TLV) format. To limit the
number of parameters that servers provide, hosts enter the parameters that they require and the server sends only those parameters.
DHCP uses the User Datagram Protocol (UDP) as its transport protocol.
The following options are commonly used in DHCP packets.
DHCP Option
Description
Subnet mask 1 — Subnet mask of the client
Router 3 — Router IP addresses that serve as the default gateway for the client
Domain name
server
6 — Domain name servers (DNS) that are available to the client
Domain name 15 — Domain name that clients use to resolve hostnames via DNS
IP address lease
time
51 — Amount of time that the client uses an assigned IP address
DHCP message
type
53:
• 1 — DHCPDISCOVER
• 2 — DHCPOFFER
• 3 — DHCPREQUEST
• 4 — DHCPDECLINE
• 5 — DHCPACK
• 6 — DHCPNACK
• 7 — DHCPRELEASE
• 8 — DHCPINFORM
Parameter request
list
55 — Parameters the server requires for DHCP clients. This is a series of octets where each octet is a DHCP
option code
Renewal time 58 — Amount of time, after the IP address is granted, that the client attempts to renew its lease with the original
server
Rebinding time 59 — Amount of time, after the IP address is granted, that the client attempts to renew its lease with any server,
if the original server does not respond
Vendor class
identifier
60 — User-defined string the Relay Agent uses to forward DHCP client packets to a specific DHCP server
180 System management