Administrator Guide

DHCP Packet Format and Options
DHCP uses the user datagram protocol (UDP) as its transport protocol.
The server listens on port 67 and transmits to port 68; the client listens on port 68 and transmits to port 67.
The configuration parameters are carried as options in the DHCP packet in Type, Length, Value (TLV) format;
many options are specified in RFC 2132. To limit the number of parameters that servers must provide, hosts
specify the parameters that they require, and the server sends only those parameters. Some common options
are shown in the following illustration.
Figure 33. DHCP packet Format
The following table lists common DHCP options.
Option Number and Description
Subnet Mask Option 1
Specifies the client’s subnet mask.
Router Option 3
Specifies the router IP addresses that may serve as the client’s default gateway.
Domain Name
Server
Option 6
Specifies the domain name servers (DNSs) that are available to the client.
Domain Name Option 15
Specifies the domain name that clients should use when resolving hostnames via DNS.
IP Address Lease
Time
Option 51
Specifies the amount of time that the client is allowed to use an assigned IP address.
DHCP Message
Type
Option 53
• 1: DHCPDISCOVER
• 2: DHCPOFFER
• 3: DHCPREQUEST
Dynamic Host Configuration Protocol (DHCP) 319