DHCP Facts

DHCP options

In addition to the IP address and subnet mask, the DHCP server can also deliver the following: DNS addresses Default router or gateway address WINS server addresses Additional TCP/IP configuration parameters

Lease

The lease is the length of time for which the assignment is valid. It contains the assigned IP address and other information for the client. Periodically, and when the client reboots, the DHCP server is contacted to renew the lease on the IP address.

DHCP lease

uses frame-level broadcasts. For this reason, DHCP requests typically do not pass through routers or other subnets. To enable DHCP across subnets: -Enable BOOTP(DHCP broadcast) requests through the router. -Configure a computer for BOOTP forwarding to request IP inofrmation on behalf of our clients.

Binding (DHCP reservation)

A binding is an association of MAC address with a specific IP address. When you create a binding, the client with the specified MAC address is assigned the same IP address each time it requests an address. For example, if you have servers that need to be accessible from outside the local network, the IP addresses of the servers should remain the same. A binding is also known as DHCP reservation.

DHCP process to obtain an IP address

1. Lease request. Initializes a ltd version of TCP/IP broadcasts DHCPDISCOVER packet requesting the location of the DHCP server. 2. Lease offer. All DHCP servers with available IP addresses send DHCPOFFER packets to client. It includes hardware address, ip address, subnet mask, duration of IP lease, and the IP address of the DHCP server making the offer. 3. Lease Selection: the client selects the IP address from first offer and broadcasts a DHCREQUEST packet to lease the IP address in that offer. 4. IP LEASE Acknowledgement: The DHCP server that made the offer responds and all other DHCP servers withdraw their offers. The address information is assigned and the offering DHCP server sends a DHCPACK(aknowledgement) packet directly to the client. The client finishes initializing and binding the TCP/IP protocol.

Interface

DHCP will compare the subnet address in the address pool with the IP address assigned to the router interfaces. The IP interface with that address will listen for and respond to DHCP requests otherwise it would be ignored. You should exclude the interface IP address from the DHCP address pool

Address pool

range of addresses that can be assigned to requesting hosts. The DHCP server only assigns address within the address pool. The DHCP server can also be configured with exclusions, which are specific addresses in the range that should not be assigned.