Abandoned lease
A lease state used when a DHCP server believes an address is already in use or has failed conflict detection. The server normally avoids offering that address until an administrator clears the state or the implementation retries it.
Category glossary
Search all termsDHCP
97 definitions associated with DHCP.
Active lease
A lease currently assigned to a client and not yet expired, released, or invalidated. The lease database normally records the address, client identifier, start time, duration, and state.
Active leasequery
A protocol mechanism that streams DHCP lease state changes to a requesting system after an initial synchronization. It is commonly used by access, security, accounting, or IPAM systems that need near-real-time binding information.
Address allocation
The process of selecting an address or prefix from a larger pool and making it available for use. Allocation does not always mean the address has been configured on a device yet.
Address conflict
A condition in which two interfaces attempt to use the same IP address on the same logical network. Symptoms can include intermittent connectivity, ARP or Neighbor Discovery instability, and DHCP declines.
Address pool
A set of addresses available for dynamic allocation. A pool is normally associated with a subnet or link and may be restricted by policies, classes, exclusions, or utilization thresholds.
Address reservation
A policy that associates a particular address with a known client identity so the server consistently offers that address. Some products distinguish a reservation from a fixed address that sits outside a dynamic pool.
ARP probe
An ARP-based check used before assigning or using an IPv4 address to determine whether another device appears to claim it. DHCP servers or clients may use probes as part of conflict detection.
Authorization
A Microsoft Active Directory control that allows approved DHCP servers to provide service in an AD domain environment. An unauthorized Windows DHCP server may stop serving clients.
Automatic allocation
An allocation method in which DHCP permanently assigns an address to a client from a pool, rather than leasing it for a limited period. Dynamic allocation is far more common.
Binding
The association between a client and configuration state, especially an assigned address or delegated prefix. A binding can include identity, lifetimes, options, and server ownership.
Binding database
The persistent or replicated data store holding leases, reservations, identifiers, lifetimes, and states. Its consistency is critical during restart and failover.
BOOTP
The Bootstrap Protocol, a predecessor to DHCP used to provide startup configuration and boot information. DHCP preserves the BOOTP message format and extends it with leases and options.
Bootstrap server
A server that supplies initial network or boot configuration to a client. Depending on context, it may be a DHCP server, TFTP server, HTTP boot server, or provisioning controller.
Broadcast
Delivery to all hosts on a local broadcast domain. DHCPv4 clients initially use broadcasts because they may not yet know their address, server, or default gateway.
Bulk leasequery
A mechanism for retrieving a large set of DHCP lease or binding data efficiently, often after a collector restarts. It is used by relay agents, access devices, accounting systems, and IPAM integrations.
Client class
A classification used to apply DHCP policy based on information such as vendor class, user class, relay information, architecture, or custom expressions. Classes can select pools, options, lease times, or access rules.
Client FQDN option
A DHCP option by which a client communicates its desired fully qualified domain name and dynamic-update preferences. The server can use it to coordinate forward and reverse DNS registration.
Client identifier
An identifier supplied by a DHCP client so the server can recognize it independently of the current link-layer address. DHCPv4 uses option 61, while DHCPv6 primarily uses a DUID.
Client state machine
The defined sequence of client states and transitions used to acquire, renew, rebind, release, or validate configuration. DHCPv4 examples include INIT, SELECTING, REQUESTING, BOUND, RENEWING, and REBINDING.
Conflict detection
A check performed before or around address assignment to see whether an address is already in use. DHCPv4 may use ICMP echo, ARP probes, or client feedback, while IPv6 clients use Duplicate Address Detection.
DDNS
Dynamic DNS, the controlled creation, modification, or deletion of DNS records without manually editing a zone file. DHCP servers and clients commonly use DNS UPDATE to register address and pointer records.
Decline
A client message indicating that an offered or assigned address appears to be in use. The server normally marks the address unavailable for investigation or later retry.
DHCID record
A DNS record that stores a digest representing a DHCP client identity and name association. It helps DHCP servers detect ownership conflicts during coordinated dynamic DNS updates.
DHCP
The Dynamic Host Configuration Protocol automatically supplies hosts with IP addressing and other configuration. DHCPv4 and DHCPv6 are separate protocols with different message formats, transports, identifiers, and state models.
DHCP client
A host or device that requests configuration from a DHCP server. The client can identify itself through a MAC address, client identifier, DUID, IAID, vendor class, or other fields depending on protocol version.
DHCP failover
A protocol or product feature that allows two DHCP servers to coordinate lease ownership and continue service after a partner failure. DHCPv4 failover behavior and DHCPv6 failover use different standards and implementations.
DHCP fingerprinting
Identification or classification of a client device based on the DHCP options it requests, sends, or orders. Fingerprints can support inventory, policy, NAC, and anomaly detection, but are not a strong identity mechanism.
DHCP lease
A time-limited authorization for a client to use an address or prefix together with associated configuration. The client renews before expiration or stops using the resource when the lease is no longer valid.
DHCP message type
A code identifying the purpose of a DHCP message, such as DISCOVER, OFFER, REQUEST, ACK, SOLICIT, ADVERTISE, or REPLY. DHCPv4 message type is carried in option 53, while DHCPv6 begins with a native message-type field.
DHCP option
A typed field that carries configuration or protocol information in a DHCP exchange. Options can provide DNS servers, routes, boot parameters, identities, relay metadata, domain search lists, or vendor data.
DHCP policy
A rule that matches client, relay, network, time, or option attributes and applies behavior such as a pool, option set, lease time, or deny action. Policy syntax and precedence are product-specific.
DHCP relay agent
A router, switch, firewall, or server function that forwards DHCP messages between clients and servers on different links. It adds link-identifying information so the server can select the correct subnet or policy.
DHCP reservation
A configuration that consistently assigns a chosen address to a known client. Matching can be based on client identifier, MAC address, DUID, or another product-supported key.
DHCP scope
A defined IPv4 subnet and its allocatable address range, exclusions, reservations, lease duration, and options. Some products instead use terms such as network, range, pool, or subnet.
DHCP server
A service that selects configuration for clients, maintains bindings, and responds to DHCP messages. It can also coordinate DNS updates, policy evaluation, failover, and lease reporting.
DHCP starvation
An attack or fault in which many fake client identities consume the available address pool, preventing legitimate clients from obtaining leases. Rate limits, DHCP snooping, port security, and monitoring help mitigate it.
DHCPv4
The DHCP protocol used to configure IPv4 hosts. It uses UDP ports 67 and 68 and commonly follows the Discover, Offer, Request, Acknowledge sequence.
DHCPv4-over-DHCPv6
A mechanism that carries DHCPv4 messages through DHCPv6 infrastructure, useful in some IPv6-oriented access networks. It is identified by DHCPv6 DHCPV4-QUERY and DHCPV4-RESPONSE messages.
DHCPv6
The DHCP protocol for IPv6. It uses UDP ports 546 and 547, DUID-based identities, identity associations, and messages such as SOLICIT, ADVERTISE, REQUEST, and REPLY.
DNS search list
An ordered set of suffixes a host may append to a relative name during local resolution. DHCP options 119 and 24 can distribute search lists for IPv4 and IPv6 respectively.
Dynamic allocation
An allocation method that leases an address to a client for a limited time from a pool. The address can be returned and later assigned to another client after release or expiration.
Dynamic range
The portion of a subnet from which a server may select addresses for ordinary lease allocation. Reservations, fixed addresses, exclusions, and static infrastructure should be planned to avoid overlap.
Enterprise number
A Private Enterprise Number assigned by IANA to an organization. DHCP vendor-specific identifiers, DUID-EN values, and many management protocols use enterprise numbers to distinguish namespaces.
Exclusion
An address or range inside a DHCP scope or subnet that the server must not lease dynamically. Exclusions are used to protect statically assigned infrastructure or address blocks managed elsewhere.
Expired lease
A lease whose valid lifetime has ended. The server may return the address or prefix to an available state after any configured hold-down, cleanup, or conflict-check process.
Failover association
A configured relationship between DHCP partners that defines peers, mode, shared address pools, timers, and state. Product implementations may call it a failover relationship, association, or peer relationship.
Failover state
The current operational condition of a DHCP failover relationship, such as normal, communications interrupted, partner down, recover, or potential conflict. Exact state names depend on the protocol and product.
Fingerprint
A pattern of observable behavior used to infer device type, software, or activity. DHCP option order, DNS query patterns, EDNS support, and TTL behavior can contribute to a fingerprint.
Fixed address
A configured mapping that assigns a specific IP address to a matching client, commonly based on MAC address, client identifier, or DUID. In Infoblox terminology, a fixed address is a managed object distinct from a DHCP reservation.
High-water mark
A configured or observed utilization threshold indicating that an address pool or resource is approaching exhaustion. Alerts should leave enough time to expand, reclaim, or redesign the pool.
Hold-down timer
A waiting period during which a resource, route, lease, or state transition is not immediately reused or reversed. Hold-downs reduce flapping and allow stale information to age out.
Hot standby
A DHCP failover mode in which one server normally owns most or all active service while the partner is ready to take over. It is often chosen when servers are in separate sites or one is intended mainly for disaster recovery.
HTTPBoot
A network boot method that retrieves boot resources using HTTP or HTTPS instead of TFTP. DHCP options identify client architecture and provide a boot URI or discovery information.
IP helper
A router or switch feature that forwards selected UDP broadcasts, most commonly DHCPv4, to configured server addresses. Because some implementations forward multiple UDP services, the exact configuration should be reviewed.
Kea
An open-source DHCPv4, DHCPv6, and dynamic DNS server developed by ISC. It uses a modular architecture, APIs, hooks, and external database options.
Lease
The time-bounded right for a client to use an address or prefix and associated configuration. Lease state is maintained by both client and server.
Lease database
The server data store containing lease bindings, states, identities, and timing. Backup, replication, integrity, and recovery of this database are central to DHCP availability.
Lease duration
The amount of time an address or prefix remains valid before renewal or expiration. Short leases increase churn and server traffic, while long leases slow reclamation and configuration change.
Lease history
A record of past address-to-client associations and state changes. It supports troubleshooting, security attribution, capacity analysis, and compliance retention.
Lease reclamation
The process of returning expired, released, or administratively cleared addresses to an available state. Reclamation may include grace periods, conflict checks, DNS cleanup, and history retention.
Leasequery
A protocol family that allows authorized systems to request current or historical binding information from DHCP servers. It is used by relays, access concentrators, security platforms, and IPAM systems.
Load balance
A failover mode or traffic design in which multiple servers actively handle clients according to a hash, split, relay selection, or load balancer. For DHCP, shared lease state and consistent client handling are critical.
Low-water mark
A threshold indicating that available address capacity has fallen below a desired reserve. It can trigger alerts, expansion, or automated allocation from a parent block.
MAC address
A link-layer hardware address, commonly 48 bits on Ethernet. DHCPv4 often uses it as part of client identity, but it can be changed, duplicated, or hidden behind relays.
Manual allocation
An allocation method in which an administrator preassigns a specific address to a client and DHCP communicates that assignment. It is commonly implemented through reservations or fixed mappings.
Maximum client lead time (MCLT)
A DHCP failover safety interval limiting how far one partner can extend a lease beyond information known to the other. It reduces the chance of duplicate allocation after communication or partner failure.
Microsoft DHCP
The Windows Server DHCP role for IPv4 and IPv6 configuration, scopes, reservations, policies, authorization, DNS updates, and DHCPv4 failover. Management is available through GUI, PowerShell, and APIs.
Next-server
A DHCP or BOOTP field or configuration identifying the server from which a client should obtain boot resources. It is commonly paired with a boot filename or URL.
Option code
The numeric identifier assigned to a DHCP option. DHCPv4 and DHCPv6 have separate option-code registries and encoding rules.
Option 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.
PAD option
A zero-length or padding option used to align or fill option areas without conveying configuration. DHCPv4 option 0 is Pad.
Partner down
A failover state in which one server is administratively or automatically authorized to assume broader responsibility because its peer is considered unavailable. Entering the state incorrectly can create duplicate allocation risk.
Per-link configuration
Configuration selected based on the client's network link, usually identified by GIADDR, link-address, interface, relay information, or subnet selection options.
Pool
A collection of resources from which a service selects a member. In DHCP it is an address range; in GSLB or load balancing it is a group of application endpoints.
Pool exhaustion
A state in which no eligible addresses or prefixes remain for new clients. Causes include genuine growth, excessive lease duration, starvation attacks, stale leases, exclusions, or a poorly sized subnet.
Private-use option
An option code range reserved for local or vendor use rather than global standardization. Private-use codes can collide when devices or organizations expect different meanings.
PXE
Preboot Execution Environment, a framework that uses DHCP and a boot transfer protocol to start a system from the network. DHCP identifies architecture, boot server, and boot file or menu information.
Relay agent
An intermediary that forwards DHCP traffic between clients and servers on different links and supplies link or subscriber context. DHCPv4 and DHCPv6 relay formats differ.
Release
A client indication that it no longer needs an assigned address or prefix. Because release messages can be lost, leases still require finite lifetimes and expiration handling.
Renew
The process by which a client asks to extend an existing binding before it expires. DHCPv4 normally starts renewal at T1, while DHCPv6 uses a RENEW message to the server that supplied the binding.
Reservation
A configured association between a client identity and a specific address or set of options. Product semantics vary regarding whether the reserved address is inside a dynamic pool and whether it creates DNS data.
Scope
A context defining which addresses, options, or policies apply. Microsoft DHCP uses scope for an IPv4 subnet and pool, while other systems may use the term for IPv6 reachability or administrative visibility.
Scope option
A DHCP option configured at a particular hierarchy level, such as server, scope, policy, reservation, or class. More-specific settings often override inherited values according to product precedence.
Scope utilization
The amount of a DHCP scope currently leased, reserved, excluded, or available. Alerts should account for addresses that are not eligible for ordinary allocation.
Server identifier
A value identifying the DHCP server participating in a transaction. DHCPv4 commonly uses option 54 with an IPv4 address, while DHCPv6 uses a DUID in OPTION_SERVERID.
Site-specific option
A DHCP option code intended for local site use. Such options can conflict across organizations and should be carefully documented when networks merge or devices move.
Split scope
A legacy DHCPv4 availability design in which two independent servers each own a non-overlapping portion of a subnet's address range, often in an 80/20 ratio. DHCP failover is generally preferred when supported.
Static mapping
A persistent client-to-address association configured by an administrator. Products may implement it as a reservation, fixed address, manual allocation, or host object.
Suboption
A typed field nested within another DHCP option, such as circuit ID inside option 82 or vendor-specific data inside an enterprise container. Its namespace is defined by the parent option.
Superscope
A Microsoft DHCP grouping of multiple IPv4 scopes on the same physical network. It supports multinets where several logical subnets share one broadcast domain, but routing and client behavior must be designed carefully.
T1
The renewal timer. DHCPv4 clients normally begin unicast renewal at T1, often 50 percent of lease time; DHCPv6 identity associations also carry T1 values.
T2
The rebinding timer. If renewal has not succeeded by T2, the client broadens its attempt to any available server, often around 87.5 percent of DHCPv4 lease time by default.
User class
Client-provided classification data used to select DHCP policy. DHCPv4 uses option 77, while DHCPv6 has OPTION_USER_CLASS.
Vendor class
Client information identifying a vendor, device family, or functional class. Servers use it for matching policies and returning vendor-specific configuration.
Vendor option
A DHCP option or nested option whose meaning is defined by a vendor or enterprise namespace. Standardized enterprise-number containers reduce collisions compared with uncoordinated private codes.
Wire format
The binary on-network representation of protocol messages, including field sizes, byte order, labels, options, and compression rules. Presentation formats such as zone files are not identical to wire format.