Not all DHCP failures look like 'no IP address'
DHCP problems fall into several distinct patterns: no lease at all, a 169.254 link-local fallback, a valid address from the wrong subnet, an exhausted scope, a lease with the wrong gateway, a lease with the wrong DNS server, or intermittent renewals. The visible symptom determines which evidence is useful.
Use AP Networking DHCP Leases and Relays Practice Test to locate the missing DHCP stage without discarding valid allocation state. The questions trace relay request and return delivery, compare scope selection with client attachment, and include pool and timer calculations with explicit assumptions.
Write down the client's full configuration before changing anything: address, prefix/mask, gateway, DNS, DHCP server if shown, and lease timing.
Scope the failure before touching the server
If every new client on one VLAN fails while clients on another VLAN obtain leases, the central server is not completely dead. Investigate that VLAN's scope, relay, forwarding path, or policy. If one client fails while neighboring new clients lease normally, focus on that endpoint, its port, or its local state before changing the entire DHCP service.
Find the last exchange that actually happened
Packet or server-log evidence can sharply narrow the problem. Discover with no Offer suggests server reachability, scope eligibility, or relay trouble. Offer with no Request shifts attention to client reception/selection. Request with no ACK points later in the transaction. A complete ACK followed by bad settings is an option/configuration problem rather than a missing-lease problem.
A server can be healthy and still have nothing to lease
Count the usable addresses in the pool, then subtract active leases, protected reservations, exclusions, and unusable/conflicted addresses as the platform reports them. Existing leases can keep working while new devices fail. That pattern is particularly important in classrooms, guest networks, and short-lease environments where client turnover is high.
Use DHCP Scope Exhaustion for the capacity math.
Remote DHCP requires a working relay path
A client begins with a broadcast on its local Layer 2 segment. If the server is on another subnet, a relay must carry the request across the routed boundary. Check that the relay is configured on the correct VLAN/interface, points to the correct server, and can reach it. Also check firewall policy for DHCP relay/server traffic rather than assuming broadcasts magically traverse routers.
A valid lease from the wrong scope is a different problem
If a client on VLAN 30 receives an address meant for VLAN 40, investigate VLAN placement, relay scope selection, or server configuration. The client may have a syntactically valid IPv4 address yet be unable to reach the intended gateway because the lease belongs to the wrong network.
Gateway and DNS options can break different layers
A bad router/default-gateway option often preserves same-subnet access while remote networks fail. A bad DNS option can preserve numeric-IP forwarding while names fail. Compare the failing lease with a working peer and identify exactly which fields differ.
Change the confirmed cause, not the entire DHCP design
- Free or expand address capacity when exhaustion is proven.
- Correct the scope or option only on the affected network when possible.
- Repair the relay path instead of moving the server unnecessarily.
- Remove duplicate/static overlap that steals addresses from the dynamic pool.
- Correct client/VLAN placement before rewriting DHCP ranges.
A DHCP fix is complete when a fresh client receives and uses the right configuration
Do not verify only by looking at the server console. Force or wait for a new lease on an affected client, confirm the assigned address belongs to the intended subnet, confirm gateway and DNS options, then test the communication that originally failed. If the issue was exhaustion, verify that the available-address count now has sustainable headroom.
Related troubleshooting deep dive
For a narrower evidence-based scenario, use DHCP DORA Troubleshooting by Stage. It includes a free interactive reasoning activity and step-by-step diagnostic evidence.