How DHCP Works: Dynamic Host Configuration Protocol Explained

Dynamic Host Configuration Protocol (alias DHCP) is a network management protocol used to automate the process of configuring devices on IP networks.

It allows them to use network services such as DNS, NTP, and any communication-based protocol in UDP or TCP. 

The DHCP server dynamically assigns an IP address and other network configuration parameters to each device on the network so that it can communicate with other IP networks.

DHCP is an improvement on an older protocol called BOOTP. DHCP is a vital part of the DDI (DNS-DHCP-IPAM) solution.

DHCP Simplifies the Management of IP addresses

DHCP is needed mainly to break down the management of IP addresses on networks. No two hosts can have similar IP addresses, and manual configuration can lead to errors.

Even in small networks, manually assigning IP addresses can be confusing, especially with mobile devices that always need IP addresses.

Additionally, most users are not technically proficient enough to locate and assign IP address information on a computer.

Automating this process makes life more straightforward for users and the network administrator.

DHCP Components

When working with DHCP, it is essential to understand all of the components. Here is their list and what they do:

• DHCP Server: A network device running a DCHP service that contains IP addresses and associated configuration information. Usually, this is a server or router, but it can be anything that acts as a host, such as an SD-WAN device.
• DHCP Client: an endpoint that receives configuration information from a DHCP server. It could be a computer, mobile device, IoT endpoint, or anything else that requires a network connection. Most are configured to accept DHCP information by default.
• IP Address Set: The range of addresses available to DHCP clients. Addresses are generally distributed sequentially from smallest to largest.
• Subnet: IP networks can be divided into segments called subnets. Subnets help manage networks.
• Location: The length of time a DHCP client retains IP address information. When the lease expires, the buyer must renew it.
• DHCP Relay: A host or router that observes messages from a client transmitted over this network and then forwards them to a configured server. The server then sends the responses to the relay agent, which delivers them to the client. This can centralize a DHCP server instead of having one server on each subnet.

Benefits of DHCP servers

Also, to simplify management, the use of DHCP servers offers other advantages. This includes:

• Correct IP Configuration: The IP address configuration parameters should be accurate, and it is easy to go wrong with entries like “192.168.159.3.” Typographical errors are often complicated to correct, and using a DHCP server reduces this risk.
• Reduced number of IP address conflicts: each connected device must possess an IP address. However, each address can only be used once, and a duplicate address will cause a conflict where one or both devices will not connect. This can occur when addresses are assigned manually, especially when there are a large number of endpoints that are only occasionally connected, such as mobile devices. Using DHCP ensures that each address is only used once.
• Automate IP address management: Without DHCP, network administrators must manually assign and revoke addresses. Tracking which device has which address can be a futile exercise, and it is nearly impossible to figure out when devices need network access and when they go. DHCP allows you to automate and centralize this so that network professionals can manage all sites from one place.
• Efficient Change Management: DHCP makes changing addresses, ranges, or endpoints easy. For example, a company may want to change its IP addressing scheme from one range to another. The DHCP server is configured with the latest information, which will be extended to the new endpoints. Also, no network configuration is required if the device is updated and replaced.

Configuration Information Sent by the DHCP Server and Critical Values

The basic flow is that the DHCP server distributes the configuration data to the requesting client according to the administrator’s rules.

Standard network settings (sometimes called “DHCP options”) include a subnet mask, router, domain name server, hostname, and domain name.

Since the requesting client does not have an IP address when it joins the network, it forwards the request.

Therefore, the protocol is used very early in IP communication. Suppose such a dynamic protocol is not used to obtain an IP address.

In that case, the client must use a predefined IP address, commonly referred to as a “static IP address,” which is manually configured on the client’s network interface configuration files or using a specific command.

The DHCP service delivers three fundamental values: 

1. Operational tasks are reduced: the network administrator no longer has to configure each client before using the network manually. 
2. The IP addressing plan is optimized: addresses are no longer used, published, and available for connecting new clients. 
3. User mobility is easy to manage: the administrator does not have to reconfigure the client when changing its network access point manually.

DHCP Lease Time Management

The IP address information designated by DHCP is only valid for a limited time, called a DHCP lease. The validity period is called the DHCP lease time.

At the expiration of the lease, the client can no longer use the IP address and must interrupt all communication with the IP network unless it requires an extension of the lease “lease” through the DHCP lease renewal cycle.

To avoid the effects of DHCP servers on downtime at the end of the rental period, customers typically begin renewing leases in the middle of the rental period. 

This recovery process ensures the full assignment of IP addresses to devices.

Any device that requests a new IP version 4 address when it arrives on the network and does not receive a response will use Automatic Private Internet Protocol (APIPA) addressing to select the address.

These addresses are in the network domain 169.254.0.0/16.

Usage Scenarios

There are four critical scenarios for using DHCP:

1. Initial connection to the client: To access network services, the client needs an IP address and other parameter values ​​from the DHCP server.
2. Extending IP usage: the client contacts the DHCP server to extend the usage of its current IP address.
3. Client connection after restart: Before restarting, the client contacts the DHCP server to confirm that it can use the same IP address.
4. Disconnect from the client: the client requests the DHCP server to release its IP address.

What are the DHCP-DHCP Options?

DHCP options can automatically supply clients with information about the network services they can use.

It is very efficient to send the IP addresses of the time server, mail server, DNS server, and print server.

It can also provide the filename and file server the client will use to perform a startup process.

It is mainly used for IP phones and Wi-Fi access points and automatically installs clients’ servers with PXE (Preboot eXecution Environment).

Implementing the DHCP Service

The Internet Systems Consortium (ISC) provides the original and most complete implementation of the DHCP service.

Compatible with IPv4 and IPv6, ISC DHCP offers a complete open-source solution to deploy DHCP servers, relay agents, and clients. Other DHCP server products include Microsoft’s DHCP server.

The DHCP service can enhance DHCP redirection to ensure high availability and traffic balancing.

ISC DHCP failover relies on two collaborative servers: a primary server (primary) and a secondary server (security).

Also, a TCP-based communication channel, called a failover channel, must be configured between two servers.