What Is DHCP?

DHCP, the Dynamic Host Configuration Protocol, is a network protocol that automatically assigns IP addresses and other network settings to devices when they join a network. DHCP removes the need to configure each device by hand, since a DHCP server hands out an IP address, subnet mask, default gateway, and Domain Name System (DNS) server addresses to every device that requests them. The Internet Engineering Task Force (IETF) defines DHCP in RFC 2131.

This article defines DHCP, explains the four-step DORA process the protocol uses, lists the settings DHCP assigns, describes leases and renewal, identifies the DHCP server in a typical network, and compares static reservations with dynamic assignment and DHCP with a manually configured static IP. Each section states one part of the topic and connects it to the automatic address assignment at the center of the protocol. The result is a complete account of what DHCP is, how the DHCP process works, and when a device uses a dynamic address instead of a fixed one.

What Is DHCP?

DHCP is a network protocol that automatically assigns IP addresses and related network settings to devices when they connect to a network. DHCP, short for Dynamic Host Configuration Protocol, runs on a server that holds a pool of available addresses and leases one to each device that requests it, defined by the IETF in RFC 2131. The defining traits of DHCP are listed below:

• Automatic assignment gives each device an IP address without manual configuration on the device.
• A central DHCP server holds a pool of addresses and leases them to requesting devices.
• A full configuration includes the IP address, subnet mask, default gateway, and DNS servers.
• A time-limited lease assigns each address for a set period, after which the device renews it.

DHCP assigns the logical address explained in the overview of an IP address, along with the subnet mask that separates the network portion from the host portion. A device can instead use a fixed address set by hand, as the guide to setting up a static IP describes.

How Does the DHCP Process Work?

The DHCP process works through four steps called DORA: Discover, Offer, Request, and Acknowledge, which a device and a DHCP server exchange to assign an address. A device joining the network has no address yet, so the device and the server complete the four-step exchange to lease one, defined in RFC 2131. The four steps of the DORA process are listed below:

1. Discover is the first step, in which the device broadcasts a DHCPDISCOVER message to find any DHCP server.
2. Offer is the second step, in which a DHCP server replies with a DHCPOFFER proposing an available address.
3. Request is the third step, in which the device broadcasts a DHCPREQUEST accepting the offered address.
4. Acknowledge is the fourth step, in which the server sends a DHCPACK confirming the lease and its settings.

The DORA exchange completes in milliseconds, so a device receives a working address within seconds of joining the network. The device sends the initial DHCPDISCOVER as a broadcast because the device has no address and does not yet know which server to contact, according to RFC 2131.

What Settings Does DHCP Assign?

DHCP assigns the IP address, the subnet mask, the default gateway, and the DNS server addresses, plus optional settings such as the lease time and domain name. A device needs more than an address to communicate, so the DHCP server delivers a complete set of parameters in the DHCPACK message. The settings DHCP assigns are listed below:

• The IP address identifies the device on the network so other devices reach it.
• The subnet mask defines which part of the address is the network and which is the host.
• The default gateway is the router address the device uses to reach other networks.
• The DNS server addresses let the device translate domain names into IP addresses.

The default gateway DHCP assigns is the router that forwards traffic to other networks, often performing the address translation in the overview of NAT. The DNS server addresses resolve names into the logical addresses defined in the overview of an IP address.

What Is a DHCP Lease?

A DHCP lease is the fixed period for which a DHCP server assigns an IP address to a device before the device must renew or release the address. A DHCP server does not give an address permanently, since the lease lets the server reclaim addresses from devices that leave the network. The traits of a DHCP lease are listed below:

• A lease duration sets how long a device keeps an address, often 24 hours on home routers.
• A renewal at 50 percent of the lease time prompts the device to extend the lease with the server.
• A rebinding stage lets the device contact any DHCP server if the original server does not respond.
• A lease release returns the address to the pool when the device disconnects or the lease expires.

A device attempts renewal at half the lease time, sending a DHCPREQUEST directly to the server that issued the address, according to RFC 2131. The lease system lets a network serve more devices over time than its address pool holds at any single moment, since departed devices return addresses for reuse.

Which Device Acts as the DHCP Server?

The home or office router usually acts as the DHCP server, while larger networks run DHCP on a dedicated server or a network appliance. A DHCP server is software that holds the address pool and answers requests, and that software runs on different hardware depending on network size. The common DHCP server locations are listed below:

• A home router runs a built-in DHCP server that assigns addresses to every device on the local network.
• A business server runs DHCP as a network service, such as Windows Server or a Linux daemon.
• A dedicated appliance handles DHCP alongside routing and firewall functions in larger networks.
• A DHCP relay agent forwards requests across subnets when the server sits on a different network segment.

A home router combines the DHCP server with the default gateway, so the same device that assigns addresses also forwards traffic to the internet. The router applies the address translation described in the explanation of NAT as it passes local traffic to the wider network.

What Is a DHCP Static Reservation?

A DHCP static reservation assigns the same IP address to a specific device every time, binding the address to the device’s hardware MAC address within the DHCP server. A static reservation keeps automatic assignment but fixes the result, so a device receives a predictable address through DHCP. The traits of a static reservation are listed below:

• A MAC address binding ties the reserved IP address to the device’s unique hardware identifier.
• A consistent address lets a printer, server, or camera keep the same IP across reconnections.
• Central management keeps the reservation on the DHCP server rather than on each device.
• No client configuration is needed, since the device still requests its address through DHCP.

A static reservation differs from a static IP set on the device, since the reservation stays under the DHCP server’s control while the device still uses the DORA process. Configuring a fixed address directly on a device instead follows the guide to setting up a static IP.

How Does DHCP Differ From a Static IP?

DHCP assigns addresses automatically and temporarily, while a static IP is a fixed address configured manually on the device and never changes. A DHCP address comes from a server and can change at renewal, whereas a static IP stays constant until an administrator edits it. The differences between DHCP and a static IP are listed below:

• DHCP assignment is automatic, requiring no configuration on the individual device.
• Static IP configuration is manual, set in the device’s network settings by an administrator.
• A DHCP address can change when the lease expires or the device rejoins the network.
• A static IP stays the same, which suits servers, printers, and port-forwarding targets.

A static IP suits a device that must keep a constant address, such as a server reached by a port forwarding rule, while DHCP suits laptops and phones that join and leave. Both approaches assign the address defined in the overview of an IP address, but DHCP automates the process that a static configuration performs by hand.

DHCP vs Static IP Comparison Table

Key Takeaways

• DHCP is a protocol that automatically assigns IP addresses and network settings to devices.
• The DORA process uses four steps: Discover, Offer, Request, and Acknowledge.
• DHCP assigns the IP address, subnet mask, default gateway, and DNS servers.
• A DHCP lease sets a time limit, after which the device renews or releases the address.
• A home router usually acts as the DHCP server for the local network.
• A static reservation fixes an address through DHCP, while a static IP is set on the device.

Last Thoughts on DHCP

DHCP is the protocol that automatically assigns IP addresses and network settings, removing the need to configure each device by hand. The DORA process, Discover, Offer, Request, and Acknowledge, leases an address within seconds, and the lease sets a time limit after which the device renews or releases the address.

A home router usually acts as the DHCP server, a static reservation fixes an address through DHCP, and a static IP set on the device suits servers that need a constant address. Readers can continue with the overview of an IP address, the guide to the subnet mask, the guide to setting up a static IP, or the guide to how networks work.