DHCP stands for Dynamic Host Configuration Protocol. It automatically assigns network settings to devices so they can communicate on a network and access the internet — without you having to manually type an IP address, subnet mask, gateway, or DNS server.
On most home networks, your router is the DHCP server.
What Does DHCP Assign?
When a device connects, DHCP hands it everything it needs to communicate:
How Does DHCP Work? (The DORA Process)
DHCP uses a 4-step handshake called DORA — good to recognize for the A+ exam:
You don't need to memorize every packet detail for A+, but understanding the overall idea — DHCP is an automated "handshake" that assigns network settings — is exactly what the exam tests.
What Is a DHCP Lease?
The server assigns an IP address for a set time period (hours or days). Before it expires, the device tries to renew it automatically. This prevents two devices from accidentally sharing the same IP.
Memory trick: Lease = "you don't own the IP forever."
How to Check DHCP on Windows
Open Command Prompt and run ipconfig /all to see full DHCP details. Look for these key lines:
Common DHCP Problems (A+ Exam Clues)
If DHCP fails, watch for these symptoms — these appear regularly in A+ scenario questions:
Common causes:
How to Troubleshoot DHCP Issues
ipconfig /release then ipconfig /renewIf you still get a 169.254 address after running /release and /renew, focus troubleshooting on the path to the DHCP server: router → switch → cable → Wi-Fi authentication.
The device is trying — the network path is blocked.
DHCP vs Static IP
Best for: Laptops, phones, classroom PCs — anything that moves or doesn't need a fixed address.
Automatic and scalable. No manual configuration needed.
Best for: Servers, printers, network gear — devices that must always be reachable at the same address.
Or use a DHCP reservation to give a dynamic device a permanent address.
DHCP Scopes, Exclusions & Reservations
These three concepts define how a DHCP server controls address assignment. They are frequently tested together on Network+ in scenario-based questions.
Exclusion: The DHCP server skips those addresses entirely — they're available for manual static assignment but DHCP will never touch them.
Reservation: The DHCP server still manages the address — but always assigns that specific IP to that specific MAC address. The device uses DHCP; it just always gets the same answer.
DHCP Relay Agents
DHCP uses broadcast packets for the Discover and Request steps. By definition, routers do not forward broadcasts — so a device on one subnet normally can't reach a DHCP server on a different subnet.
A DHCP relay agent (also called an IP helper) solves this. It's configured on a router interface and listens for DHCP broadcasts from clients. When it hears one, it converts it to a unicast packet and forwards it to the DHCP server's IP address. The server responds to the relay, which forwards the reply back to the client.
Without relay agent: Client (VLAN 10) ──broadcast──► Router ✗ drops it DHCP Server (VLAN 20) With relay agent (ip helper-address configured on router interface): Client (VLAN 10) ──broadcast──► Router ──unicast──► DHCP Server (VLAN 20) ◄──unicast── Client ◄──unicast── Router
On Cisco routers, the relay agent is configured per interface with ip helper-address [DHCP server IP]. This is the most commonly tested implementation detail for Network+.
The relay agent forwards not just DHCP traffic but also several other broadcast-based protocols by default — DNS, TFTP, NTP, and others on their standard ports.
"Devices on VLAN 20 are getting APIPA addresses even though a DHCP server exists on VLAN 10. What is the most likely cause?"
Answer: No DHCP relay agent configured on the router interface for VLAN 20. The DHCP broadcasts from VLAN 20 clients are being dropped at the router and never reaching the server.
APIPA — When DHCP Fails
If a device sends DHCP Discover messages and receives no response, it doesn't give up entirely. Windows automatically assigns itself an address in the 169.254.0.0/16 range — this is called APIPA (Automatic Private IP Addressing).
Range: 169.254.0.0 – 169.254.255.255 (/16) Assigned: Automatically by the OS when DHCP fails Scope: Link-local only — same subnet communication only No gateway: Cannot route to the internet or other subnets Symptom: Device has a 169.254.x.x address → DHCP failed Fix: Restore DHCP server or run ipconfig /release then ipconfig /renew
An APIPA address on a Windows device is almost always a symptom of a DHCP problem — the DHCP server is down, unreachable, or the scope is exhausted. Two devices with APIPA addresses in the same subnet can still talk to each other, but neither can reach the internet or any other subnet.
Rogue DHCP Servers
A rogue DHCP server is an unauthorised device on the network that responds to DHCP Discover messages. Because DHCP is first-come-first-served, if the rogue server responds faster than the legitimate one, it wins — and can hand out false IP addresses, a wrong default gateway (routing traffic through an attacker's machine), or a malicious DNS server (redirecting web traffic).
What the attacker controls: Default gateway (enabling man-in-the-middle), DNS server (enabling phishing via DNS hijacking), or simply an incorrect subnet — causing a denial of service.
Defence — DHCP snooping: A managed switch feature that designates only certain ports as "trusted" DHCP sources. Any DHCP Offer or Acknowledge arriving on an untrusted port is dropped. This is the primary mitigation and is tested on Network+.
Also tested: On Windows Server networks, the DHCP server must be authorised in Active Directory before it can respond to clients — a built-in rogue server prevention mechanism.
Final Summary
Lock In DHCP Quickly
Take the practice exams, then come back and review the troubleshooting section.
Related Networking Articles
Preparing for the A+ Exam?
See the books, practice exams, and free resources that actually work.