What is DHCP (Dynamic Host Configuration Protocol)? – Definition

Dynamic Host Configuration Protocol (DHCP) is a protocol used by networked devices or clients to obtain the parameters necessary for operationin an Internet Protocol network. This protocol reduces system administration workload, allowing devices to be added to the network with little or no manual configurations. Dynamic Host Configuration Protocol is a way to administrator network parameter assignment from a single DHCP server, or a group of DHCP servers arranged in a fault-tolerant manner. Even in small networks,

Dynamic Host Configuration Protocol is useful because it can make it easy to add new machines to the local network.

DHCP is also recommended even in the case of servers whose addresses rarely change, so that if a server needs to be readdressed, changes can be made in as few places as possible. DHCP can beused to directly assign addresses to servers and desktop machines, and, through a Point-to-Point Protocol (PPP) proxy, to dialup and broadband on-demand hosts, as well as for residential Network address translation (NAT) gateways and routers. DHCP is generally not appropriate for infrastructure such as non-edge routers and DNS servers.

The Dynamic Host Configuration Protocol (DHCP) provides a framework for passing configuration information to hosts on a TCP/IP network. DHCP is based on the BOOTP protocol, adding the capability of automatic allocation of reusable network addresses and additional configuration options.

DHCP consists of two components :

• A protocol that delivers host-specific configuration parameters from a DHCP server to a host.

• A mechanism for the allocation of temporary or permanent network addresses to hosts.

IP requires the setting of many parameters within the protocol implementation software. Because IP can be used on many dissimilar kinds of network hardware, values for those parameters cannot be guessed at or assumed to have correct defaults. The use of a distributed address allocation scheme based on a polling/defense mechanism, for discovery of network addresses already in use, cannot guarantee unique network addresses because hosts might not always be able to defend their network addresses.

DHCP supports three mechanisms for IP address allocation :

Automatic Allocation : DHCP assigns a permanent IP address to the host.

Dynamic Allocation : DHCP assigns an IP address for a limited period of time. Such a network address is called a lease. This is the only mechanism that allows automatic reuse of addresses that are no longer needed by the host to which it was assigned.

Manual Allocation : The host’s address is assigned by a network administrator.

Wherever possible, DHCP-assigned addresses should be dynamically linked to a secure DNS server, to allow troubleshooting by name rather than by a potentially unknown address. Effective DHCP-DNS linkage requires having a file of either MAC addresses or local names that will be sent to DNS that uniquely identifies physical hosts, IP addresses, and other parameters such as the default gateway, subnet mask, and IP addresses of DNS servers from a DHCP server. The DHCP server ensures that all IP addresses are unique, i.e., no IP address is assigned to a second client while the first client’s assignment is valid.

How DHCP Works 

As its name indicates, DHCP provides dynamic IP address assignment. What this means is that instead of having to rely on a specific IP address, a computer will be assigned one that is available from a subnet or “pool” that is assigned to the network. DHCP also extends BOOTP functionality to provide IP addresses that expire. BOOTP indirectly uses a form of leasing that never expired, but the term wasn’t actually used until the introduction of DHCP. When DHCP assigns an IP address, it actually leases the identifier to the host computer for a specific amount of time. The default lease is five days, but a network administrator should evaluate their own particular circumstances to determine an appropriate lease.

In basic terms, the DHCP lease process works as follows:

• A network device attempts to connect to the Internet.

• The network requests an IP address.

• The DHCP server allocates (leases) the network device an IP address, which is  forwarded to the network by a router.

• DHCP updates the appropriate network servers with the IP address and other configuration information.

• The network device accepts the IP address.

• The IP address lease expires.

• DHCP either reallocates the IP address or leases one that is available.

• The network device is no longer connected to the Internet.

• The IP address becomes an available address in the network pool of IP addresses.

• A client’s DHCP-issued address expires, the client will Attempt to obtain a new address by broadcasting.

To set up DHCP, you basically need a DHCP-supported client (at least one) and router, and a DHCP server. The client is a computer or other device on a network that requires an IP address and or other network configuration information. The router functions as a forwarding (or routing) agent of IP address requests from the DHCP server. The DHCP server is key to the entire operation. It is responsible for allocating, leasing, reallocating, and renewing IP addresses. Windows and Linux both support DHCP software.