Classless static route option
DHCP option 121, which supplies IPv4 routes as destination prefixes and next-hop routers. When present, clients generally use it instead of the older classful static route option 33.
Category glossary
Search all termsDHCPv4
45 definitions associated with DHCPv4.
DHCPACK
A server message confirming the offered or renewed IPv4 configuration. The client can enter or remain in the BOUND state after validating and applying the configuration.
DHCPDECLINE
A client message reporting that the offered address appears to be in use. The server normally marks the address as conflicted or abandoned.
DHCPDISCOVER
The initial client message used to locate DHCP servers and request configuration. It is often broadcast because the client does not yet have a usable IPv4 address.
DHCPFORCERENEW
A server-initiated message asking a client to begin lease renewal so configuration can be refreshed. Authentication or trusted network controls are important because the message can otherwise be abused.
DHCPINFORM
A message sent by a client that already has an IPv4 address but wants additional local configuration options. The server replies with DHCPACK without allocating a new address lease.
DHCPNAK
A negative acknowledgement indicating that the client's requested or remembered IPv4 configuration is not valid for the current network or cannot be honored. The client normally returns to initial configuration.
DHCPOFFER
A server proposal containing an available IPv4 address and configuration. A client can receive multiple offers and select one by sending DHCPREQUEST.
DHCPRELEASE
A client message telling the server that it is voluntarily giving up its lease. Because release is not guaranteed during shutdown or disconnect, servers must still rely on lease expiration.
DHCPREQUEST
A client message used to select an offer, confirm a remembered address, renew a lease, or rebind. Context and included options tell the server which use applies.
DORA
A mnemonic for the common DHCPv4 address-acquisition exchange: Discover, Offer, Request, Acknowledge. Additional messages and paths exist for renewal, reboot, decline, release, and information-only operation.
GIADDR
The gateway IP address field inserted by a DHCP relay agent. The server uses it to identify the client link and select the appropriate subnet, pool, and options.
Option 119
The Domain Search option, which distributes an ordered list of DNS search suffixes using DNS name compression encoding. It is distinct from option 15, which carries a single domain name.
Option 12
The Host Name option, used by a DHCPv4 client to provide a hostname string. Modern DDNS workflows often prefer the Client FQDN option because it carries a complete name and update flags.
Option 121
The Classless Static Route option, which distributes destination prefixes and next-hop routers. It supersedes the older classful static route option for capable clients.
Option 125
The V-I Vendor-Specific Information option, which namespaces vendor data using IANA enterprise numbers. It supports multiple vendors in a structured format.
Option 15
The Domain Name option, used to provide a client's local DNS domain. It is not the same as the domain search list option, which can carry multiple suffixes.
Option 150
A commonly deployed Cisco-specific option used to provide one or more TFTP server addresses to IP phones and other devices. It is not a generally assigned IETF DHCP option in the same sense as standard options.
Option 160
A code often used by vendors for provisioning URLs or server discovery. Its meaning is not universal and must be verified for the specific client and server implementation.
Option 161
The Manufacturer Usage Descriptions URL option for DHCPv4, allowing a device to provide a URL for its MUD policy description.
Option 162
The DHCPv4 Discovery of Network-designated Resolvers option, used to advertise encrypted DNS resolver information to clients.
Option 43
The Vendor-Specific Information option, whose internal format is defined by the vendor or application. It is widely used for controller discovery and provisioning.
Option 50
The Requested IP Address option, used by a client to identify the IPv4 address it wants during selection or reboot processing.
Option 51
The IP Address Lease Time option, carrying the requested or granted lease duration in seconds.
Option 52
The Option Overload option, indicating that DHCP options also appear in the BOOTP file or server-name fields because the normal options area was insufficient.
Option 53
The DHCP Message Type option, identifying messages such as DISCOVER, OFFER, REQUEST, DECLINE, ACK, NAK, RELEASE, and INFORM.
Option 54
The Server Identifier option, normally containing the selected DHCP server's IPv4 address. Clients use it to identify which offer they accepted and where unicast requests should go.
Option 55
The Parameter Request List, an ordered list of option codes the client asks the server to provide. Its contents and ordering are often used for DHCP fingerprinting.
Option 60
The Vendor Class Identifier option, used by a client to identify its vendor type, device family, or boot environment. Servers can match it to apply vendor-specific policy.
Option 61
The Client Identifier option, used to identify a client independently of the BOOTP hardware-address field. Servers should apply consistent identity rules to avoid duplicate leases.
Option 66
The TFTP Server Name option, historically used to tell a booting client where to obtain files. Modern environments may use other boot-server or URL options.
Option 67
The Bootfile Name option, identifying the initial file a network-boot client should request. The value depends on architecture, firmware, and provisioning system.
Option 77
The User Class option, which lets a client present one or more administrator-defined class values. Servers can use it for policy and option selection.
Option 81
The Client FQDN option, used to communicate a client's FQDN and dynamic DNS update preferences. It supports coordinated ownership of A and PTR updates.
Option 82
The Relay Agent Information option, added by a relay or access device to identify circuit, subscriber, remote device, link selection, or other access context. Servers use its suboptions for policy and address selection.
Option 90
The DHCP Authentication option. Deployment is limited, and network-layer controls such as DHCP snooping are more common in enterprise access networks.
Option 93
The Client System Architecture Type option, used by PXE and network boot systems to select a boot program appropriate for BIOS, UEFI, x86, ARM, and other architectures.
Relay Agent Information option
The full name of DHCP option 82. Its suboptions can identify circuit, remote device, subscriber, link selection, and other relay-supplied context.
Remote ID
An option 82 suboption identifying the remote access device, subscriber unit, or relay context. The exact value format is implementation-specific.
Renewing state
The client state entered at T1 when it attempts to renew its lease directly with the original server. If renewal fails until T2, the client enters REBINDING.
Router option
DHCP option 3, which supplies one or more default gateway addresses in order of preference. It should not be confused with classless static route option 121.
SIADDR
The BOOTP/DHCP "next server" address field, historically used to identify a server involved in the client's bootstrap process. Modern DHCP boot behavior may instead use options.
Subnet selection option
An option that lets a client or relay identify the subnet from which an address should be selected when GIADDR or the receiving interface is insufficient.
XID
The transaction identifier in the BOOTP/DHCPv4 header used to match client and server messages. It is selected by the client and repeated through the exchange.
YIADDR
The "your IP address" field in a BOOTP/DHCPv4 reply, containing the IPv4 address offered or assigned to the client when applicable.