O flag
The Other Configuration flag in a Router Advertisement, indicating that hosts should use DHCPv6 for information other than addresses, such as DNS or domain search data. Host behavior is implementation-dependent.
Read definitionAlphabetical glossary
O glossary terms
42 definitions beginning with O.
42 terms
Obsolete
A registry or specification status indicating that a protocol element should no longer be used for new implementations or deployments. Obsolete code points may remain documented for compatibility and packet analysis.
Read definitionODoH
Oblivious DNS over HTTPS separates the client's network identity from its DNS query by using a proxy and an encrypted message for a target resolver. The proxy sees the client but not plaintext query content, while the target sees the query but not the original client address.
Read definitionOffline signing
A model in which zone data is signed outside the serving authoritative server, then transferred or published as a signed zone. It can isolate private keys but adds publication and freshness complexity.
Read definitionOn-link
Reachable directly on a local link without sending through a router. IPv6 hosts learn on-link prefixes from Router Advertisements and Neighbor Discovery rules.
Read definitionOpen resolver
A recursive resolver that accepts queries from arbitrary Internet clients. Open resolvers are vulnerable to abuse for reflection attacks, policy bypass, and unauthorized resource consumption.
Read definitionOperational technology (OT)
Industrial, building, medical, or process-control systems that interact with physical operations. OT DDI designs often emphasize deterministic change control, long device lifecycles, isolated networks, and legacy protocols.
Read definitionOPT record
A pseudo-resource-record used to carry EDNS metadata in the additional section of a DNS message. Its class field advertises UDP payload size, while TTL bits carry extended RCODE, version, and flags.
Read definitionOpt-out
A feature, most notably NSEC3 opt-out, that lets a signed delegation-centric zone omit some insecure delegations from the authenticated chain. It improves scalability but weakens proof granularity around those delegations.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 125
The V-I Vendor-Specific Information option, which namespaces vendor data using IANA enterprise numbers. It supports multiple vendors in a structured format.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 161
The Manufacturer Usage Descriptions URL option for DHCPv4, allowing a device to provide a URL for its MUD policy description.
Read definitionOption 162
The DHCPv4 Discovery of Network-designated Resolvers option, used to advertise encrypted DNS resolver information to clients.
Read definitionOption 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.
Read definitionOption 50
The Requested IP Address option, used by a client to identify the IPv4 address it wants during selection or reboot processing.
Read definitionOption 51
The IP Address Lease Time option, carrying the requested or granted lease duration in seconds.
Read definitionOption 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.
Read definitionOption 53
The DHCP Message Type option, identifying messages such as DISCOVER, OFFER, REQUEST, DECLINE, ACK, NAK, RELEASE, and INFORM.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 67
The Bootfile Name option, identifying the initial file a network-boot client should request. The value depends on architecture, firmware, and provisioning system.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 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.
Read definitionOption 90
The DHCP Authentication option. Deployment is limited, and network-layer controls such as DHCP snooping are more common in enterprise access networks.
Read definitionOption 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.
Read definitionOption code
The numeric identifier assigned to a DHCP option. DHCPv4 and DHCPv6 have separate option-code registries and encoding rules.
Read definitionOption Request Option (ORO)
A DHCPv6 option containing the option codes a client wants the server to return. It is analogous to the DHCPv4 Parameter Request List.
Read definitionOption space
A namespace or grouping that defines how option codes and suboptions are interpreted. Products may support standard, vendor, relay, site-specific, or custom option spaces.
Read definitionORO
Abbreviation for Option Request Option. A client lists desired configuration option codes in OPTION_ORO.
Read definitionOrphaned record
A DNS or IPAM object whose intended owner, target, device, or application no longer exists or cannot be identified. Orphans create security risk and complicate address reclamation.
Read definitionOut-of-bailiwick
A name outside the relevant zone or delegation boundary. An out-of-bailiwick child name-server name normally does not require parent glue because it can be resolved through its own delegation path.
Read definitionOverlapping address space
Use of the same IP prefixes in separate routing or tenant domains. IPAM must track the associated VRF, network view, tenant, or context so identical addresses remain distinguishable.
Read definitionOwner name
The domain name at which a resource record set is stored. It is the left-hand name in zone-file presentation format.
Read definition