2 DHCP Configuration and Troubleshooting

How DHCP assigns network settings

Dynamic Host Configuration Protocol (DHCP) automatically assigns clients IP addresses and other network settings. A DHCP server can serve clients on its directly connected subnet or, with a relay, serve clients on other subnets.

Plan scopes and address ranges

Create a separate DHCP scope for each client subnet. Before activating a scope, confirm that its subnet mask and address range are correct, that the range does not conflict with another DHCP server, and that manually assigned infrastructure addresses are excluded from dynamic allocation.

A subnet ID identifies the scope; it is not a client address. For example, the subnet ID 192.168.20.0 identifies a scope for the 192.168.20.0/24 client subnet. One possible plan uses 192.168.20.1 for the gateway, 192.168.20.10 for DNS, and addresses from 192.168.20.50 through 192.168.20.200 for DHCP clients.

A scope can provide the default gateway (option 3), DNS servers (option 6), and DNS domain name (option 15). Set options at the scope level when they differ by subnet; server-level options apply more broadly.

Configure a Windows DHCP scope

This Windows Server PowerShell example creates a scope, excludes addresses for manually configured devices, sets common options and an eight-day lease duration, and reserves an address for a DHCP-configured printer. Replace the example addresses and MAC address with values from the network. In Windows DHCP, the reservation address must be within the configured scope distribution range.

The example scope covers 192.168.20.1 through 192.168.20.254; it excludes 192.168.20.1 through 192.168.20.49 and reserves 192.168.20.60 for the printer.

PowerShell example

Manage leases and reservations

A DHCP lease is a time-limited address assignment. Clients attempt to renew a lease before it expires. If a client cannot reach the original server, it may later broadcast a request to any available DHCP server.

Choose the lease duration to fit address capacity and client turnover. Shorter leases can recycle addresses faster in a high-turnover network, while longer leases reduce renewal traffic on stable networks.

A DHCP reservation associates a specific address with a client identifier, commonly the network interface’s MAC address. The device still uses DHCP but normally receives the same reserved address, which is useful when a device needs a predictable address and should remain centrally managed. For a device configured with a manual static address instead, keep that address outside the dynamic pool or exclude it.

Relay requests across subnets

DHCP discovery begins as a local broadcast, which routers normally do not forward. When clients and the DHCP server are on different subnets, configure a DHCP relay agent, often called an IP helper, on the client VLAN’s router interface or Layer 3 switch interface. Configure it to forward DHCP requests to the server.

The relay supplies information identifying the client subnet, allowing the server to select the correct scope. Confirm that the relay can reach the server and that the server has an active scope for the relayed subnet.

Read the DHCP exchange

The typical IPv4 DHCP exchange is DORA: Discover → Offer → Request → Acknowledgment. The point where the exchange stops helps narrow the problem. No Discover suggests a client, link, VLAN, or local broadcast issue. A Discover with no Offer points toward the server, relay, scope, or filtering. An Offer followed by failure to complete may indicate a competing server or a client/server configuration problem.

DHCP uses UDP ports 67 and 68. Packet captures can show whether each message reaches its next hop.

Troubleshoot DHCP systematically

Check the client and local network first. Verify that the adapter is enabled, the cable or wireless connection works, and the client is on the intended VLAN. On Windows, run ipconfig /all and confirm that DHCP is enabled and the assigned address, mask, gateway, and DNS servers are expected. An address beginning 169.254 commonly indicates that the client did not obtain a DHCP lease.

Next, check the server and scope. Confirm that the DHCP service is running, the scope is active and matches the client subnet, and free addresses remain. Check the lease database for relevant conflicts and verify that scope options specify the correct gateway and DNS servers.

Look for BAD_ADDRESS entries and check whether manually configured devices are using addresses inside the dynamic pool without exclusions.

For a remote subnet, verify that the relay is enabled on the correct client-facing interface, points to the correct server address, and can reach that server. Confirm that the server has a scope for the relayed subnet.

In an Active Directory domain, verify that the Windows DHCP server is authorized. Also check DHCP policies, filters, firewall rules, and whether another service or device is interfering with DHCP traffic.

After correcting a client-side issue, run ipconfig /release followed by ipconfig /renew. If the problem persists, capture DHCP traffic and identify whether Discover, Offer, Request, and Acknowledgment messages are sent and received.