Dynamic Host Control Protocol (DHCP)

Posted by Unknown Posted on Wednesday, February 13, 2013

Introduction to DHCP


Dynamic Host Control Protocol (DHCP) is responsible for assign TCP/IP network configuration to workstation on a private network. The server on a network becomes DHCP server when you install and configure this protocol.  So once you installed the DHCP service, the server will be responsible for providing network configuration to the client machines or devices.

When you enable "Obtain an IP address automatically" in the TCP/IP configuration on the workstation, the DHCP Server takes over the responsibility of assigning the TCP/IP parameters. So you may think what if there is no DHCP server on a network?

A Windows-based computer that is configured to use DHCP can automatically assign IP address to itself  if a DHCP server is not available or does not exist on the network. It is called APIPA (Automatic Private IP Addressing). This feature can be used in small Local Area Network without any difficulty. The Internet Assigned Numbers Authority (IANA) has reserved 169.254.0.0-169.254.255.255 for Automatic Private IP Addressing. As a result, APIPA provides an address that is guaranteed not to conflict with routable addresses.

What is the need of DHCP server on a Private network?


The DHCP server is commonly used in a domain network and the overall purpose of DHCP is to reduce the amount of time required to configure and administer a network.

Assign static IP address is not difficult if your network is limited with 5 or 10 computers but imagine if you have network with more than 100 computers. It will be very hard and also there is chance to have address conflicts. Here DHCP prevents possible address conflicts and human errors. The DHCP Server automatically provides each LAN client with an IP address, a default gateway (router) and DNS server IP addresses

How Does DHCP Work?


Administrator creates pools of addresses and other configurations (Gateway, DNS server etc…) in sever and the DHCP server dynamically assigns IP address and these configurations to the client on demand with lease time. The server can reassign address after lease expires or client can extend lease time dynamically.

When DHCP client boot, it broadcasts DISCOVER packet on local subnet and DHCP servers send OFFER packet with lease information.  DHCP client selects lease and broadcasts REQUEST packet and selected DHCP server sends ACKNOLEDGMENT packet. This is how client obtain IP address other configuration from the DHCP server. This process simply called DORA.

DHCP


Preparing for DHCP


Before you going to implement DHCP server, you must need to plan your network or else it will put you in trouble. While planning the network, make sure that the DHCP Server always remains accessible for the DHCP clients or provide a backup DHCP Server for times when the primary DHCP Server cannot be reached

You should be aware of below terms when you install DHCP server

Scope: - Scope is a valid range of IP addresses which are available for assignments or lease to client computers on a particular subnet. In a DHCP server, you configure a scope to determine the address pool of IPs which the server can provide to DHCP clients.

Super Scope: - A super scope is an administrative feature of DHCP servers that you can you can group multiple scopes as a single administrative entity. With this feature, a DHCP server can support DHCP clients on a single physical network segment (such as a single Ethernet LAN segment) where multiple logical IP networks are used. When more than one logical IP network is used on each physical subnet or network, such configurations are often called multinets.

Split scope: - This added feature in Windows server 2008 to provide a load balance or create a backup DHCP server.

Address Pool: - An address pool is a list of IP addresses which are available to the network clients. Each IP pool is defined by a starting IP address and an ending IP address. For an example starting IP address of 192.168.0.10 and the ending IP address of 192.168.0.20 represents 11 IP addresses.

Address Leases: - The DHCP lease is the length of time that the DHCP Server allows the client to use a particular assigned IP address.  When the client determines that it is about to expire, it sends a request to the server asking to increase the lease time. The server extends the lease if the DHCP policy allows it. If server not allowed or did not respond to the request, the client must restart the discovery process.

Reservations: Reservations are permanent lease assignments that are used to ensure that a specified client on a subnet can always use the same IP address.

Server options: - Options configured at the server level are applied to all DHCP clients, irrespective of the subnet on which they reside. Any options that you want to apply to all DHCP clients should be configured at this level, eg: if you wanted to configure all clients on the network to use the same DNS server, you would configure this option at server level.

Scope options: - If you want to configure DHCP options to apply only to DHCP clients on a specific subnet, you should configure the options at the scope level, eg: the IP address of the default gateway for a subnet should be configured at the scope level.