Dynamic Host Control Protocol

A network interface card.

Dynamic Host Control Protocol (DHCP) is the automated process used to assign Internet Protocol (IP) addresses to the devices on a computer network. DHCP was developed as a replacement for the Bootstrap Protocol (BOOTP) and operates similar to this protocol while reducing the burden required to administrate DHCP enabled networks. Once a DHCP server is configured and authorized to serve on a computer network, the process of assigning IP addresses requires no administrative intervention as long as the DHCP server is properly functioning.

DHCP: Simplifying IP Address Management

Typical connections to network switch.

The DHCP protocol provides a significant benefit over other methods of assigning IP addresses by reducing the amount of time and planning necessary for IP address management. DHCP provides central management of IP addresses and eliminates the need to manual configure IP addresses for the devices within a computer network. With manual, or static, IP addressing, computer administrators are required to manually configure each device on a computer network IP address. By using the DHCP protocol on a DHCP server, devices on the computer network obtain addresses from the server automatically and administrators have no need to manually configure individual computer IP addresses.

Reducing IP Addressing Errors Using DHCP

Manual configuration of IP addresses increases the possibility of errors that result from assigning an IP address to multiple devices on the network or from inadvertently assigning an invalid IP address. A DHCP server that has been properly configured on a network all but eliminates the possibility of these types of errors occurring. With DHCP, unless the DHCP server software makes a mistake, an unlikely event, the IP address assigned to each device will be unique to that device.

IP Address Assignment for Mobile Users

A multiple computer Network.

As the popularity of mobile connectivity devices has increased, DHCP provides another benefit over statically configured networks. On occasion, a person visiting may have need to access the intranet or other resources on the network. On a static IP network, the user's device would require modifications to the configuration of the device to enable connectivity. DHCP configured networks allow for connection to the network with no requirement to alter the configuration of the user's device.

DHCP Address Pool

For a network to use DHCP addressing, the network requires that a DHCP server is enabled on the network. Most home network routers have the DHCP server built into the system and provide DHCP functions automatically. A network without a DHCP enabled connection device requires the installation, configuration and authorization of the DHCP server to begin using DHCP.

DHCP software requires the configuration of the IP address pool, the subnet mask and the default gateway (optional) to begin assigning IP addresses. The address pool is the range of addresses that a DHCP server uses to assign IP addresses to network devices. A sample address pool would be similar in form to the following: starting IP - 192.168.0.10, ending IP - 192.168.0. 100. When a DHCP server receives a broadcast request from a network device to obtain an IP address, a network DHCP server assigns, or "leases," the device an address from within this IP address pool.

DHCP Address Leasing Process

The process of obtaining an IP address using DHCP and a DHCP server begins with the powering of a network enabled device. The Network Interface Card (NIC) of the device detects a network connection and sends a broadcast over the network containing a DHCP discover packet. In response, every DHCP enabled server on the network segment responds to the DHCP request with an IP address from the pool of available IP addresses while also removing this IP address from the pool of available IP addresses. The DHCP address response from each DHCP server includes the IP address, the subnet mask, the IP address of the DHCP server and the specified time limit that the lease for the IP address remains active.

The device requesting the IP address receives the response from the DHCP servers and accepts the IP address from the first server to respond, and it broadcasts a message to the DHCP servers that an IP address has been accepted. This broadcast serves two functions: to let the DHCP server that owns the accepted IP address know that it has accepted the IP address and to let additional DHCP servers on the network know that they may return the IP addresses they sent back to their IP address pool for use on other devices requesting IP addresses.

The selected DHCP server then responds back to the device to acknowledge the message and relay additional information concerning the network, such as the the default gateway and the IP address of the Domain Name System (DNS) server.

References

• “Network+ Guide to Networks”; Tamara Dean; 2010