What is a DHCP Server and the Things that You Need to Know About it

DHCP (Dynamic Host Configuration Protocol), is a standardized network protocol used to automate the process of assigning IP addresses and other configuration parameters, such as subnet masks, default gateways, DNS servers, and others, to devices on a network. The primary purpose of DHCP is to simplify the management of IP addresses within a network. It eliminates the need for manual IP address assignment, which reduces the likelihood of errors, thus making network management and scaling more efficient.

How do DHCP servers work?

We call the DHCP step-by-step process DORA. It’s a four-step process that includes, “Discover, Offer, Request, and Acknowledge”. Let’s talk more about it:

1. Discover(D). When you connect your device to the internet, it broadcasts a discovery message to all devices on the network. The broadcast nature of this message ensures that it reaches potential DHCP servers. It’s like it’s saying I need an IP address and the necessary configurations to be able to connect to the network.

2. Offer(O). Upon receiving the Discover message, DHCP servers on the network evaluate the request. If a server determines that it can fulfill the client’s request for an IP address and other configuration parameters, it responds with a DHCP Offer. This Offer includes an available IP address, lease duration, subnet mask, default gateway, DNS server, and any other relevant configuration details.

3. Request (R): After receiving one or more DHCP Offers, the client decides which offer to accept. It then sends a DHCP Request message to the chosen DHCP server, indicating its acceptance of the offered configuration.

4. Acknowledge (A): The DHCP server that receives the Request message acknowledges the client’s acceptance by sending a DHCP Acknowledge (ACK) message. This message finalizes the lease agreement, and the client can now use the provided IP address and configurations

During the offer stage, the client decides which offer to accept based on its internal criteria, such as the first offer received or the offer with the best configuration.

This DORA process ensures a systematic and organized way for devices to obtain the necessary network configuration information from DHCP servers.

IP Address Leasing

Speaking of lease agreements in the final stage of DORA. Yes, DHCP has a fundamental aspect known as IP address leasing. The IP address is leased to the device, such as your smartphone or computer, for a specific duration, which is defined by the DHCP server but is a configurable parameter, so a network administrator can also set how long a certain device is going to use that IP address. 

Now, before the lease expires, the device can request a renewal from the DHCP server.  If the server approves, the lease is extended, allowing the device to keep using the same IP address.  If the device no longer needs the IP address or leaves the network, it can send a DHCP Release message to inform the server. Alternatively, the lease may simply expire.

IP address leasing is a very effective method to manage available IP addresses within a network, especially in environments with a large number of devices that come and go, as these processes take place within the network without the need for manual intervention.

Here’s also a video tutorial for those who prefer to listen and watch:

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