Dynamic Host Configuration Protocol or DHCP is defined as an IP network protocol that relies on client-server architecture to automatically set IP addresses and other attributes to an IP host to enable information transfer between network nodes. This article explains how DHCP works and its key features.
Table of Contents
- What Is DHCP (Dynamic Host Configuration Protocol)?
- How Does DHCP Work?
- Features of DHCP
What Is DHCP (Dynamic Host Configuration Protocol)?
Dynamic Host Configuration Protocol or DHCP is an IP network protocol that relies on client-server architecture to automatically set IP addresses and other attributes to an IP host to enable information transfer between network nodes.
Dynamic Host Configuration Protocol (DHCP) is a protocol used by devices linked to the internet to guide the distribution and use of IP addresses. The internet exists heavily regulated by a series of guidelines, principles, and standards generally called protocols. All these protocols are standardized by the IETF (Internet Engineering Task Force). These public standards are critical because they ensure that devices and programs, irrespective of who created them, are compatible with others worldwide.
The Dynamic Host Configuration Protocol is not a program like any other protocol. It exists as a set of standards that lays out the procedures for requesting and sharing IP addresses over a computer network. The DHCP is used when creating address distribution functions.
Understanding what is DHCP
Dynamic Host Configuration Protocol (DHCP) interfaces between a server and client automatically designate an Internet Protocol address and other data to an Internet Protocol host. The DHCP makes it possible for a host to get the necessary Transmission Control Protocol (TCP/IP) configuration information from the server.
This DHCP server is the server for the given network and can assign IP addresses to the computers interacting on that network automatically. This automatic control of the designation of IP address is a function of the DHCP standard in response to the queries broadcasted by client computers.
DHCP is a network management standard employed in giving out IP addresses to computers and any other device or nodes on a network to foster better communication. Before using DHCP, network administrators were tasked with manually assigning Internet Protocol addresses to all the peripheral devices on a network, exposing the system to errors and placing a tremendous burden on the admins, especially in large networks.
Presently, admins can use the DHCP system in large networks such as those used by campuses, enterprises, Wide Area Network (WAN), etc., and in smaller networks such as residential networks.
DHCP network comprises two components, the central server with the DHCP component installed on it and client instances represented by computers or any internet-enabled device connected to the network. Even when devices change location, DHCP assigns new IP addresses. The DHCP standard has been integrated and can be used in versions 4 and 6 of the Internet Protocol.
See More: What Is Network Management? Definition, Key Components, and Best Practices
Importance of DHCP
Internet Protocol (IP) address is a vital component that one must assign to all devices operating in a Transmission Control Protocol/ Internet Protocol network. DHCP is a set of rules that makes this process easier and less cumbersome.
With or without DHCP, one must assign IP addresses to devices because they contain the information used to accurately direct the transmission of data packets passed across any network. Without DHCP, computers moved to another network will have to undergo manual configuration to assign them new IP addresses. Similarly, the IP addresses assigned to computers that have left the network must be manually retrieved.
DHCP, however, does this entire process automatically. The DHCP server keeps a pool of available IP addresses from which it leases an address to any client on its radar. When using DHCP, the addresses are leased; they are temporarily assigned to a client rather than permanently associated with that device. This dynamic nature of the leased address means that anyone not in use is immediately returned to the pool and can be given to another device.
Benefits of the DHCP Network
Using DHCP, organizations can expect the following benefits:
- Internet Service Providers (ISP) use DHCP to assign IP addresses to their users. This method of IP address designation is suitable as not all users are connected to the internet throughout the day. Consequently, addresses can be assigned for the duration of a connection and retained in the pool once the connection is severed.
- DHCP reduces the occurrence of IP conflict to the barest minimum. Because IP addresses are centrally assigned, there are lesser chances of two devices giving the same address or errors in the address.
- DHCP also provides support and easy transition for devices still using the BOOTP standard, which was in place before the Dynamic Host Configuration Protocol.
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How Does DHCP Work?
To fully understand the working of DHCP, we must look at the components of the DHCP network:
- DHCP server: This is the central device that holds, assigns, and manages IP addresses. It can be a server, router, or SD-WAN appliance.
- DHCP client: This is the endpoint that requests for IP addresses and can be installed on any type of peripheral device, although most are part of the default settings.
- Subnets: These are parts of a more extensive network.
- DHCP relay: This refers to devices like routers that acts as a middleman between clients and server, amplifying the messages to reach their destination goal.
The overall process and detailed mechanisms explain the working principle of Dynamic Host Configuration Protocol (DHCP). A DHCP system consists of two essential elements: the client and the server.
The clients are peripheral devices, while the DHCP server allocates IP addresses. The physical server often comes with a backup. Other devices function similarly to servers, such as SD-WAN appliances or the more common wireless access points.
A device that tries to access a DHCP-enabled network first sends a request for an IP address. Because the device has no IP address initially, it adds its MAC address to the message it sends in search of a server. This address identifies the network card of the device. The request sent by the client reaches the DHCP server, which does not hesitate to send an available address to the device. Further, the server monitors the use of the address to return it to the address pool if it is out of use, or the allocated time has expired.
It is natural to wonder how the end device initially connects to the server without an IP address, which is explained by an intricate system of exchanging messages and acknowledgments. To start, all modern devices have a DHCP client system installed during manufacturing, which is enabled by default.
The DHCP client is present in peripheral devices and computers and starts functioning as soon as the computer is turned on, and the operating system is running. Therefore, most devices can already find and connect to a DHCP network.
The entire process, although a bit complex, occurs automatically within seconds. The initialization process involves four message types which are:
1. DHCP discovery
The discovery message is the first message transmitted across the network to which clients are linked. The message type DHCPDISCOVER is sent widely across the network and not to a specific address, as the client is unaware of the server’s address.
The discovery message is a packet with a detailed destination (usually 255.255.255.255), showing that the client is part of that network. The packet may also contain a specific subnet broadcast address if configured. The discovery operation is a universal procedure that can fit into any DHCP server, provided the client is in that network.
Although there are no fixed destination addresses for individual servers and clients, the port number is a fixed parameter used in all DHCP communication between servers and clients. DHCP servers have a User Datagram Protocol (UDP) port number of 67, so listen for messages addressed to this port number. On the other hand, DHCP clients have the UDP port number 68 and only respond to messages sent to number 68.
2. DHCP offer
The DHCP is the reply sent by the server after receiving the discovery message. The message type is DHCPOFFER, which is broadcasted widely across the network using the UDP port number 68 so that any DHCP client connected to that network can pick it up. However, the message is targeted to just one client, and the server does this by attaching the MAC address of a specific client. Other clients ignore the message when they come across a non-self MAC address.
Included in the DHCP offer is an IP address that a client might accept using. The message also tells clients about the lease period for the DNS server addresses, the IP address, the IP address of the server, the default gateway, and the subnet mask. All this information ensures that the device is fully integrated into the network.
3. DHCP request
The DHCPREQUEST is a protocol that safeguards and guides the client in a network with multiple servers. Some networks, typically large ones, can have multiple servers, all capable of receiving the discovery message and sending out an offer to the client with an IP address. Because this is a possibility, the DHCP client is structured to send out a request message after receiving an offer which may be the first of many offers.
The DHCP request message confirms the choice of the client and usage of the IP address in the offer it received. The request message is transmitted with the server’s IP address embedded in the chosen offer message. The server that sent out the offer message chosen then receives the request message and certifies that the client’s IP address is unavailable for other devices. If other servers send out offer messages, they will return the offered IP addresses to their pool of addresses while waiting for another device that may need it.
4. DHCP acknowledgment
DHCP acknowledgment is the final step in the initialization process. It is a message sent by the server that supplied the IP address. The message is defined as “DHCPACK”, acknowledging that the IP address in question has been successfully leased to the client. The configuration is complete at this stage, and the client has a new, functional IP setting.
5. Control of lease time
DHCP is a dynamic protocol because it does not assign permanent IP addresses to the clients. While this may be perfect for some devices, DHCP attaches a specific lease time to each IP address. Once this period is up, the client can no longer use the address and is removed from the network. The concept of lease time serves to eliminate inactive clients.
For clients operating in the network, the lease is renewed halfway through the time, so the user does not experience any downtime. On the other hand, an inactive user cannot renew the lease and is removed from the network. Devices that are shut down also have their lease terminated immediately to increase the pool of available addresses.
See More: What Is Network Hardware? Definition, Architecture, Challenges, and Best Practices
Features of DHCP
The latest DHCP server versions are equipped with critical features that increase its functionality as an IP management platform. These features include:
1. Request For Comments (RFC) compliance
The RFC is a document that describes the protocols, technologies, and standards that have guided the internet since its inception. The DHCP standard has three major management features that it supports and complies with the RFC. They are:
- Scope: A scope describes the many IP addresses from one sub-network. The DHCP server uses scope to allocate IP addresses to every network member. For example, the scope for a network with ID number 192.167.1.0 can have IP addresses ranging from 192.167.1.1 to 192.167.1.155. Any client in that network can be assigned an IP address from that group.
- Super-scope: The super scope is simply an aggregate of more than one scope. It enables the users to have many clients connected to the same physical segment.
- DHCP options: The DHCP options refer to specific non-mandatory parameters that the server can assign to clients on the network. DHCP options include the server, scope, class, and reserved client options. As already established, functions like the IP address, subnet mask, etc., are always sent to the clients. However, other functions may or may not be assigned, such as the WINS server, among others. They are called DHCP options.
2. Domain Name System (DNS) integration
It is imperative for DHCP networks also to have DNS integration. DHCP concomitant with DNS integration makes it possible for clients using earlier versions of Windows or clients using other OS than Windows to update their records automatically.
3. Active Directory integration
Active Directory integration is another feature of the DHCP standard that enables DHCP servers to be authorized within Active Directory. Active Directory integration helps sieve unauthorized servers and maintain a system of only authorized servers within the network. This way, the administrator will fully control the servers allowed in the network.
One disadvantage of this feature is that it works only with Windows 2000 DHCP servers. The implication is that any other version of the DHCP server introduced in the network can escape the Active Directory authorization process.
4. Microsoft vendor-specific options
Microsoft vendor-specific options are also a feature supported by DHCP. For example, it is possible to disable NetBIOS over Transmission Control Protocol (TCP/IP), also called NetBT. This option is available only on Windows 2000 DHCP and allows the clients to enable or disable it. The ability to disable NetBIOS does not exist in early versions and creates a considerable improvement in the recent versions.
This is because disabling it reduces the network’s broadcast traffic. Further, DHCP release is the feature used to regulate the calling back of DHCP lease from DHCP clients when the device shuts down.
5. Microsoft support for multicast IP address allocation
Multicast Address Dynamic Client Allocation Protocol (MADCAP) is supported by Windows 2000 DHCP protocol. Multicast clients use the MADCAP to become a part of multicast groups. You should note that the multicast scope is independent of the DHCP scope, meaning that multicast clients can receive the IP address that is part of the DHCP scope and the multicast address as part of a different scope.
Other features/networking parameters of the DHCP network that forms a part of the DHCP networking functionality and provide efficiency and security include the following;
- Default Gateway, which transfers data from one network or sub-network to another.
- A subnet mask separates the IP address into the host and network address sections.
- DNS server, which translates domain names (letters) to the form of IP addresses (numbers).
See More:
Takeaway
DHCP is a foundational technology powering the modern internet. It assigns the IP address, subnet mask details, default gateway, etc., to the IP host, which makes it possible to relay information from one endpoint to the other on the world wide web. Even though it was first defined in 1993, it remains a necessary protocol, constantly updated for IPv4, IPv6, and future versions.
By knowing how DHCP works, organizations can strengthen their networking capabilities and also protect them against security vulnerabilities that DHCP could potentially cause.
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