DHCP

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Introduction[edit]

DHCP means "Dynamic Host Configuration Protocol". Its basic function is to enable individual computers on an IP network to take out their configurations from a server. Here the servers mean particularly, a DHCP server or servers having no precise information about the individual computers until they request the information. Use of this protocol eases the burden to administer a large IP network and facilitates devices to be added to the network with little or no manual intervention. The most vital information provided via this network application protocol is the IP address. Devices called DHCP clients acquire configuration information for operation in an Internet Protocol network. Dynamic Host Configuration Protocol automates network parameter assignment to network devices from one or multiple, fault-tolerant DHCP servers. Even in small networks, DHCP is helpful since it can make it easy to add new machines to the network.

Technical Overview[edit]

DHCP necessitates disk storage or some other type of reliable non-volatile storage for making the DHCP service more compatible with servers as compared to dedicated routers. The large-scale routers are most likely to have no DHCP server function. However there are few types of servers which can be configured to route and serve DHCP. The DHCP RFC specially says that DHCP is not intended for use in configuring routers. When a DHCP-configured client i.e. a computer or any other network-aware device gets connected to a network, the DHCP client sends a broadcast query requesting essential information from a DHCP server. The DHCP server manages a pool of IP addresses and information about client configuration parameters such as default gateway, domain name, the DNS servers, other servers such as time servers, and so forth. On receiving a valid request, the server assigns the computer an IP address, a DHCP lease i.e. length of time the allocation is valid), and other IP configuration parameters, such as the subnet mask and the default gateway. The query is usually initiated instantly after booting, and must complete before the client can initiate IP-based communication with other hosts.

IP Address[edit]

It is a number typically written as four numbers separated by periods for e.g., 107.4.1.3 or 84.2.1.111. An IP address is also known as an IP number. It distinctively identifies a computer which is using Internet. It is equivalent to a telephone number. As a telephone number is used by the telephone network for direct calling to us, in a similar manner an IP address is used by the Internet to direct data to our computer. DHCP helps out in getting a functional and unique IP number into the hands of the computers that make use of the Internet.

DHCP lease[edit]

It is the amount of time granted by DHCP server to the DHCP client permission for using a particular IP address. A typical server permits its administrator to set the lease time.

Allocation Methods[edit]

Based on implementation, the DHCP server allocates IP-addresses in following three ways.

  1. Dynamic allocation: In this method, a network administrator assigns a range of IP addresses to DHCP, and each client computer on the LAN has its IP software configured to request an IP address from the DHCP server during network initialization. The request-and-grant process uses a lease concept with a controllable time periods, allowing the DHCP server to reclaim and then reallocate IP addresses which are not renewed i.e. the dynamic re-use of IP addresses.
  2. Automatic allocation: In this method, the DHCP server permanently assigns a free IP address to a requesting client from the range defined by the administrator. This is similar to dynamic allocation, but the DHCP server keeps a table of past IP address assignments, so that it can preferentially assign the same IP address to a client that the client previously had.
  3. Static allocation: In this method, the DHCP server allocates an IP address based on a table with MAC address/IP address pairs, which are manually filled in. Only requesting clients with a MAC address listed in this table will be allocated an IP address. This feature is not supported by all routers and is differently called Static DHCP Assignment by DD-WRT, fixed-address by the dhcp documentation, DHCP reservation or Static DHCP (by Cisco/Linksys and IP reservation or MAC/IP binding by various other router manufacturers.

Other Methods of Allocation[edit]

  • Manual allocation: In this method, the DHCP server does not assign the IP address; instead, the client is configured with a user-specified static IP address.
  • Autoconfiguration: Address autoconfiguration of a link-local IP address is used when a host is unable to obtain an IP address by any other method.