DHCP glossary

What is DHCP Reservation?

A DHCP reservation is an addressing configuration that binds a specific IP address to a client identifier, such as a MAC address, on a DHCP server. It lets the server hand the same address to that device on every request while the address stays managed centrally and is excluded from the general lease pool.

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A DHCP reservation is a server-side configuration that binds a specific IP address to a client identifier (typically a MAC address), ensuring a device receives the same address on every DHCP request while keeping that address managed centrally and excluded from the dynamic lease pool. This approach enables organizations to assign fixed addresses to servers, printers, cameras, and network appliances without manual configuration on each device, centralizing address management and allowing updates like DNS or gateway changes to propagate automatically through lease renewal. However, operational risks arise from poor tracking, including address conflicts when reserved addresses fall within the dynamic range, stale entries for retired hardware, MAC address spoofing vulnerabilities, and failover issues in redundant server environments, all of which require careful IPAM practices and security controls to mitigate.

  • Reservations must be placed outside the dynamic pool range to prevent address conflicts, and configuration errors such as incorrect MAC addresses or changed hardware can cause devices to receive unintended pool addresses instead of their reserved IPs.
  • Unlike static IP addresses configured manually on devices, DHCP reservations are managed server-side and remain effective across relay agents as long as the server correctly identifies the client’s subnet and the identifier matches what the client actually sends.
  • Reservation records should be tracked in IPAM alongside device owner and purpose information, and MAC address–based reservations require pairing with additional security controls such as port security or network access control since MAC addresses can be spoofed.

DHCP reservation at a glance

AttributeValue
CategoryDHCP server configuration
Defined inDynamic Host Configuration Protocol, RFC 2131, which defines the client identifier and requested address mechanics; reservations themselves are a server feature rather than a protocol message
Protocol and portDHCP over UDP, server port 67 and client port 68
OSI layerApplication layer
Primary useGiving servers, printers and other fixed-role devices a predictable address without manual configuration on the device
Related termsDHCP, DHCP lease, DHCP scope, static IP address, IP address management, MAC address

How DHCP reservation works

A reservation is a record stored on the DHCP server that links one client identifier to one IP address within a scope. The client still uses the normal DHCP exchange and is unaware that a reservation exists.

  1. Create the reservation: An administrator enters the client identifier, usually the MAC address or a DHCP client ID, and the IP address to assign. The address must belong to the scope's subnet.
  2. Client sends a discover: When the device starts or renews, it broadcasts a DHCP discover message that includes its hardware address and, if used, a client identifier option.
  3. Server matches the identifier: The server compares the identifier in the request with its reservation records. A match overrides selection from the dynamic pool.
  4. Server offers the reserved address: The server offers the reserved address, and the client requests and receives it through the usual request and acknowledgment messages.
  5. Client renews on the lease: The client holds the address under a normal lease and renews it at the usual timers, receiving the same address each time.

Reserved addresses are still leased, so lease times, options such as default gateway and DNS servers, and scope settings still apply. Many servers let a reservation carry its own options that override scope-level values. If the identifier changes, as when a network card is replaced, the device no longer matches and receives a pool address until the reservation is updated.

Why DHCP reservation matters in enterprise networks

Reservations let organizations keep fixed addresses for printers, servers, cameras and network appliances while managing them in the same place as every other address. Compared with static configuration on each device, this removes the need to touch the device, keeps address data in one authoritative system and lets changes such as a new gateway or DNS server propagate through lease renewal.

Operational problems appear when reservations are not tracked. A reserved address placed inside the dynamic pool range can be offered to another client, producing an address conflict. Duplicate reservations for the same address, or stale entries for retired hardware, waste space and cause confusion during troubleshooting. In IPAM, reservations should be recorded alongside the device owner and purpose so that they can be audited and reclaimed.

On the security side, a reservation based on a MAC address is a convenience, not authentication. MAC addresses can be spoofed, so a client that copies the identifier receives the reserved address and any firewall or access rule tied to it. Pair reservations with controls such as port security or network access control when an address carries privileges.

DHCP reservation vs Static IP address

A reservation is assigned by the DHCP server through the normal lease process, while a static IP address is configured manually on the device and the DHCP server does not know about it. What is Static IP address?

AttributeDHCP reservationStatic IP address
Where the address is setOn the DHCP serverOn the device itself
Client behaviorClient runs DHCP and receives the address in a leaseClient does not use DHCP for that interface
Changing the addressEdit the record and let the client renew or restartReconfigure each device individually
Other options such as gateway and DNSDelivered by the server and updated centrallyEntered by hand and updated by hand
Conflict riskLower, because the server tracks the address as reservedHigher if the address is not excluded from the pool or recorded in IPAM

DHCP reservation vs DHCP Lease

A lease is the time-limited grant of an address to a client, while a reservation is the server rule that ensures a particular client always receives the same address in that grant. What is DHCP Lease?

AttributeDHCP reservationDHCP Lease
What it isA configured binding of identifier to addressA temporary assignment with a duration
Created byAn administratorThe server when it answers a client request
ExpirationDoes not expire until removedExpires unless the client renews it
RelationshipDetermines which address the lease containsStill applies to a reserved client and is renewed normally

Common DHCP reservation issues

If a reserved address falls within the dynamic range and the server does not exclude it, another client may be offered it before the reserved device comes online. The result is an address conflict, often seen as intermittent connectivity loss on both devices.

A typo in the MAC address, or a device that sends a client ID option instead of using its hardware address, means the record never matches. The device receives an address from the pool and the reservation appears to be ignored.

Replacing a network card, a failed printer or a virtual machine with a regenerated MAC address leaves the old record in place. The new device gets a pool address while the reserved address sits unused until someone updates the record.

Two records that use the same IP address or the same identifier within or across scopes can cause rejected configuration or unpredictable assignments. This often occurs when reservations are entered by hand in several places without a central record.

If reservations are defined on only one server in a failover or split-scope pair, the partner server hands out pool addresses when it answers. Clients then change address after a failover event, and DNS or firewall rules tied to the old address stop working.

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DHCP reservation FAQ

Related terms

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DHCP Scope

A DHCP scope defines the range of IP addresses and related configuration settings that a DHCP server can assign to devices on a network segment.

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DHCP Lease

A DHCP lease is the period during which a DHCP server assigns an IP address to a client device, allowing it to use that address before renewal or expiration.

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Static IP address

A static IP address is an IP address that doesn’t change and was manually configured for a device, rather than assigned by a DHCP server.

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IP Address Conflict

An IP address conflict occurs when two or more devices on a network are assigned the same IP address, disrupting network communication.

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DHCP Relay Agent

A DHCP relay agent forwards DHCP messages between clients and servers on different network subnets, enabling centralized DHCP services.

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IPAM (IP address management)

IP address management (IPAM) is a method for planning, tracking, and managing IP address space on a network.

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DHCP (Dynamic Host Configuration Protocol)

DHCP (Dynamic Host Configuration Protocol) is the standard mechanism to dynamically assign IP addresses within a network.

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Dynamic IP Address

A dynamic IP address is an IP address assigned to a device that changes over time, unlike a static IP address that remains constant.

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