DHCP (Dynamic Host Configuration Protocol)
Course Contents
1. Introduction to DHCP
Imagine if you had to manually set an IP address, subnet mask, gateway, and DNS servers on your computer every time you joined a network. It would quickly become tedious and prone to mistakes.
DHCP (Dynamic Host Configuration Protocol) is a network protocol that solves this issue by automatically assigns all necessary network settings. This means you can connect to a network and start communicating immediately without manual input.
What does DHCP provide?
When your device connects to a network, DHCP typically assigns:
- An IP address: Identifies your device on the network
- A subnet mask: Defines the local network range
- A default gateway: Allows communication with external networks, including the internet
- DNS servers: Resolves domain names like google.com into IP addresses
This automatic process ensures that devices such as computers, smartphones, and printers are correctly configured without human intervention.
Understanding DHCP in Action
When your device (the DHCP client) connects to a network, it sends out a request to find a DHCP server which is usually your router, or a dedicated server in larger networks.
The server responds by assigning all the necessary IP settings for a specific time period, called a lease.
Even though everything happens automatically, you can still check whether your device is using DHCP.
How to Verify DHCP on Windows
Method 1: Through the Control Panel
- Open the Control Panel and go to Network and Sharing Center.
- Click on your active connection. In my case, it’s labeled Ethernet 3 because I’m connected via an Ethernet cable.
- Select Properties.
4. In the list, select Internet Protocol Version 4 (TCP/IPv4).
5. Make sure both options are set to:
- “Obtain DNS server address automatically”
- “Obtain an IP address automatically”
If both are selected, DHCP is enabled.
Method 2: Using Command Prompt
You can also check if DHCP is enabled using the ipconfig /all command.
Open Command Prompt and type:
C:\Users\PingMyNetwork>ipconfig /all /Output Omitted/ Ethernet adapter Ethernet 3: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Realtek USB GbE Family Controller #2 Physical Address. . . . . . . . . : 38-7C-76-11-B0-70 DHCP Enabled. . . . . . . . . . . : Yes Autoconfiguration Enabled . . . . : Yes IPv4 Address. . . . . . . . . . . : 192.168.1.19(Preferred) Subnet Mask . . . . . . . . . . . : 255.255.255.0 Lease Obtained. . . . . . . . . . : Monday, January 27, 2025 10:47:39 PM Lease Expires . . . . . . . . . . : Tuesday, January 28, 2025 10:47:39 AM Default Gateway . . . . . . . . . : 192.168.1.254 DHCP Server . . . . . . . . . . . : 192.168.1.254 DNS Servers . . . . . . . . . . . : 192.168.1.254 NetBIOS over Tcpip. . . . . . . . : Enabled /Output Omitted/
What to Look For
- DHCP Enabled: Yes
This confirms that your device uses DHCP to obtain its configuration. - DHCP Server
This shows the IP address of the server that assigned your settings (often your home router).
- IPv4 Address, Subnet Mask, Default Gateway
These are the parameters your device received automatically.
- Lease Obtained / Lease Expires
These lines indicate the duration of the DHCP lease.
What is a DHCP Lease?
- Lease Obtained: When the DHCP server assigned the IP address.
- Lease Expires: When the address is no longer valid unless renewed.
When the lease expires, the client must request a new lease. If not, the IP is returned to the available pool for reuse by other devices.
This lease system prevents unused IPs from staying assigned forever. It helps the DHCP server manage addresses efficiently, especially on networks with many clients.
2. Understanding the DHCP Process
We’ve seen that DHCP automatically assigns IP settings to devices.
But how does it actually work behind the scenes?
Let’s break it down together.
DHCP Ports
The DHCP protocol uses specific ports to communicate:
- UDP port 68 — used by the DHCP client
- UDP port 67 — used by the DHCP server
These ports are used throughout the DHCP process.
Make sure to remember them for your CCNA exam.
The DHCP Process (Step by Step)
When a device connects to a network without an IP address, it uses DHCP to automatically obtain its network configuration.
This happens through a 4-step process often referred to as DORA:
Discover, Offer, Request, Acknowledge
Let’s go through each step.
Step 1 — DHCP Discover
When a device joins a network for the first time, it has no IP address and doesn’t know where the DHCP server is.
To find one, it sends a DHCP Discover message as a broadcast on the network.
This way, any DHCP server listening can respond with an offer.
In this capture, we can observe:
- Destination MAC: FF:FF:FF:FF:FF:FF — unknown server MAC, so broadcast.
- Source IP: 0.0.0.0 — the client doesn’t have an IP address yet.
- Destination IP: 255.255.255.255 — broadcast to the entire subnet.
- UDP Ports: source port 68 (client), destination port 67 (server).
This broadcast is essential because the client has no prior information about the server’s address.
Step 2 — DHCP Offer
When a DHCP server receives the Discover message, it replies with a DHCP Offer, proposing an IP address and configuration settings to the client.
This message is usually sent as a unicast directly to the client’s MAC address.
In this capture, we can observe:
- Source IP: DHCP server IP (192.168.1.1)
- Destination IP: proposed IP for the client (192.168.1.11)
- Destination MAC: MAC address of the client (learned from Discover)
- UDP Ports: source port 67 (server), destination port 68 (client)
- Your (Client) IP Address field: proposed IP (192.168.1.11)
- DHCP Options:
- Option 51: Lease time
- Option 6: DNS servers
- Option 3: Default gateway
The client can now decide whether to accept the proposed configuration.
Step 3 — DHCP Request
After receiving the Offer, the client replies with a DHCP Request to confirm it wants to use the proposed IP address.
This message is broadcast to inform all DHCP servers that one has been selected.
In this capture, we can observe:
- Source IP: 0.0.0.0 — the client still doesn’t have an IP
- Destination IP: 255.255.255.255 — broadcast
- Destination MAC: FF:FF:FF:FF:FF:FF — broadcast
- UDP Ports: source port 68 (client), destination port 67 (server)
- DHCP Options:
- Option 50: Requested IP address (192.168.1.11)
- Option 54: DHCP Server Identifier (192.168.1.1)
This message tells the server:
“I accept your offer. Please assign me this IP.”
Other DHCP servers that may have sent an offer now know their proposal was declined.
Step 4 — DHCP ACK
Once the client has requested the offered IP, the DHCP server replies with a DHCP ACK to finalize the process.
This message confirms that the IP address is officially assigned and provides all necessary configuration parameters.
In this capture, we can observe:
- Source IP: 192.168.1.1 — the DHCP server
- Destination IP: 192.168.1.11 — the client’s new IP
- Destination MAC: the client’s MAC address (unicast)
- UDP Ports: source port 67 (server), destination port 68 (client)
- DHCP Options:
- Option 51: Lease duration
- Option 6: DNS servers
- Option 3: Default gateway
Once the client receives this ACK, it can officially use the assigned IP address.
It is now fully configured and ready to join the network.
3. DHCP DORA
The DHCP process follows four key steps, known by the acronym DORA:
- Discover — The client sends a broadcast to find any available DHCP server.
- Offer — The server responds with an IP address and network details.
- Request — The client formally requests to use the offered IP.
- Acknowledge — The server confirms the lease and finalizes the configuration.
Each step is essential for automatic IP configuration. Make sure you understand them well, this is a common topic on the CCNA exam.
4. Conclusion
In this lesson, you discovered how DHCP automates IP address assignment in a network.
You learned:
- How to verify DHCP settings on a device
- How the DORA process works (Discover, Offer, Request, Acknowledge)
- The UDP ports used: 67 (server) and 68 (client)
- How to analyze DHCP traffic with Wireshark
What’s Next?
In the next course, we’ll see how to configure a Cisco router as a DHCP server.
Let’s move from theory to practice!