• Glossary
  • What is DHCPACK (Dynamic Host Configuration Protocol acknowledgment)?

DHCP glossary

What is DHCPACK (Dynamic Host Configuration Protocol acknowledgment)?

DHCPACK (Dynamic Host Configuration Protocol acknowledgment) is a DHCP message type that a server sends to confirm an address lease and deliver the final configuration parameters to a client. It answers a DHCPREQUEST or DHCPINFORM, as defined in RFC 2131, and gives the client the lease terms to apply.

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DHCPACK is the Dynamic Host Configuration Protocol acknowledgment message that a DHCP server sends to confirm an address lease and deliver final configuration parameters to a client, as defined in RFC 2131. It represents the final message in the standard four-message DHCP exchange and is the moment when a lease becomes valid, allowing a device to gain working network access. When DHCPACK messages are slow or missing due to causes such as exhausted address pools, misconfigured relay agents, server failures, or firewall blocking, clients experience network outages and may fall back to link-local addressing, making accurate logging and monitoring of DHCPACK activity essential for network operators to detect pool exhaustion and troubleshoot connectivity issues.

  • DHCPACK is sent in response to a DHCPREQUEST or DHCPINFORM message and carries the assigned address, lease time, and configuration options that the client must apply to use the network.
  • Common failure scenarios preventing DHCPACK delivery include misconfigured relay agents with incorrect giaddr values, firewall or DHCP snooping rules blocking server replies, overloaded servers delaying acknowledgment during mass client events, and rogue DHCP servers answering requests before the legitimate server.
  • Monitoring DHCPACK and DHCPNAK message activity provides operators with visibility into lease issuance rates and early indicators of address pool exhaustion, supporting both IPAM systems and post-incident investigation of network outages.

DHCPACK at a glance

AttributeValue
Full nameDynamic Host Configuration Protocol acknowledgment
CategoryDHCP message type
Defined inRFC 2131 for DHCP, with message type values carried in the option format of RFC 2132
Protocol and portUDP, server port 67 and client port 68
OSI layerLayer 7 (application)
Primary useConfirming a lease and delivering configuration parameters to a client
Related termsDHCP, DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPNAK, DHCP lease

How DHCPACK works

DHCPACK is the last message in the standard four-message DHCP exchange for a new lease. It is also used when a client renews a lease or asks only for configuration parameters.

  1. Discover: The client broadcasts a DHCPDISCOVER to find available DHCP servers.
  2. Offer: Each responding server sends a DHCPOFFER with a proposed address and configuration options.
  3. Request: The client picks one offer and broadcasts a DHCPREQUEST naming the chosen server, which also tells the other servers their offers are declined.
  4. Acknowledge: The chosen server records the lease and sends a DHCPACK carrying the address, lease time and options. The client can then configure its interface and begin using the address.

Before using the address, a client may probe the network, for example with ARP, to check that no other host holds it. If the address is already in use, RFC 2131 has the client send a DHCPDECLINE and start over. If the server cannot honor a request, it sends a DHCPNAK instead of a DHCPACK. A DHCPACK also answers a DHCPINFORM, in which case it carries options but no address.

Why DHCPACK matters in enterprise networks

The DHCPACK is the point at which a lease becomes valid, so it is the moment a device gains working network access. In large networks, servers send these messages continuously as devices join, roam and renew. RFC 2131 has clients begin renewing at half the lease time by default, so lease length directly sets how many DHCPACK messages a server must produce.

When DHCPACK messages are slow or missing, clients stay without addresses or fall back to link-local addressing, and users see this as a network outage. Common causes include exhausted address pools, relay agents that are misconfigured, a failed server with no failover partner, or conflicts that lead clients to send DHCPDECLINE. Logging DHCPACK and DHCPNAK activity gives operators an accurate view of lease issuance and the early signs of pool exhaustion.

DHCPACK messages are not authenticated in basic DHCP, so a rogue server on the same segment can answer a request and hand out false gateway or DNS settings. Switch features such as DHCP snooping limit this by allowing server replies only from trusted ports. Accurate records of which DHCPACK issued which address also support IPAM and incident investigation.

DHCPACK vs DHCPNACK

DHCPACK confirms a lease and delivers configuration, while DHCPNAK, the message name in RFC 2131, tells the client its requested address cannot be used and it must start over. What is DHCPNACK?

AttributeDHCPACKDHCPNACK
PurposeConfirms the lease and supplies final parametersRejects the client's request for an address
Sent in reply toDHCPREQUEST or DHCPINFORMDHCPREQUEST
Client action on receiptConfigures the interface and uses the leaseStops using the address and restarts discovery
Typical triggerValid request for an address the server can grantClient requests an address that is wrong for its subnet or no longer available

DHCPACK vs DHCPOFFER

DHCPOFFER proposes an address and options before the client commits, while DHCPACK is the final confirmation that makes the lease valid. What is DHCPOFFER?

AttributeDHCPACKDHCPOFFER
Position in exchangeLast message of the lease exchangeSecond message, after DHCPDISCOVER
Sent in reply toDHCPREQUEST or DHCPINFORMDHCPDISCOVER
Binds the leaseYes, the lease takes effectNo, the proposal is only an offer
Number sentOne, from the selected serverOne from each responding server

Common DHCPACK issues

Clients on a remote subnet send requests but never receive a DHCPACK because the relay agent has a wrong or missing giaddr, or return traffic is blocked. Server logs show an acknowledgment sent while the client logs show repeated requests.

After a power event or a Wi-Fi roam storm, many clients request at once and an overloaded server or slow backend database delays the DHCPACK. Clients time out, retry, and may fall back to link-local addresses.

An unauthorized DHCP server, often a consumer router plugged into a switch, sends its own DHCPACK with a wrong gateway or DNS server. Users get addresses that work locally but cannot reach other networks.

A client probes the address after the DHCPACK, finds it already in use, and declines the lease. This is usually caused by a static device inside the dynamic range, and it shows up as repeated declines in server logs.

Security policy or DHCP snooping on an untrusted port discards the server's DHCPACK, so the server records a lease that the client never learns about. Check that the server-facing port is trusted and that UDP ports 67 and 68 are permitted.

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DHCPACK FAQ

Related terms

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DHCP (Dynamic Host Configuration Protocol)

The standard mechanism to dynamically assign IP addresses within a network.

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DORA process

The sequence of discover, offer, request, and acknowledge messages exchanged between client and server for DHCP.

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DHCPDISCOVER

Sent by the client as the first step of the DHCP client/server interaction to discover what servers exist.

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DHCPOFFER

Sent by the server to the client in response to a DHCPDISCOVER to offer an IP address, lease time, and network configuration parameters.

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DHCPREQUEST

Sent by the client to the server in response to a DHCPOFFER indicating which server the client has chosen to further interact with.

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DHCPNACK

Sent by the server to the client in response to a DHCPREQUEST indicating that the server does not acknowledge the request.

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IPAM (IP address management)

A method for planning, tracking, and managing IP address space on a network.

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DDI (DNS, DHCP, and IPAM)

The integration of DNS, DHCP, and IPAM into one management solution.

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