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DHCP August 10, 2005

Posted by Coolguy in Networks.
  • Dynamic Host Configuration Protocol is a protocol for assigning dynamic IP addresses to devices on a network
  • With dynamic addressing, a device can have a different IP address every time it connects to the network. In some systems, the device’s IP address can even change while it is still connected.
  • Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than requiring an administrator to manage the task.
  • This means that a new computer can be added to a network without the hassle of manually assigning it a unique IP address. Many ISPs use dynamic IP addressing.
  • DHCP allows a computer to join an IP-based network without having a pre-configured IP address. DHCP is a protocol that assigns unique IP addresses to devices, then releases and renews these addresses as devices leave and re-join the network
  • DHCP environments require a DHCP server set up with the appropriate configuration parameters for the given network
  • Devices running DHCP client software can then automatically retrieve these settings from DHCP servers as needed
  • When a computer logs on to the network, the DHCP server selects an IP address from a master list and assigns it to the system.
  • When a DHCP client device attaches itself to the network for the first time, it broadcasts a DHCPDISCOVER packet. A DHCP servers on the local segment will see the broadcast and return a DHCPOFFER packet that contains an IP address and other information
  • The client may receive multiple DHCPOFFER packets from any number of servers, so it must choose between them, and broadcast a DHCPREQUEST packet that identifies the explicit server and lease offer that it likes the best
  • This decision may be based on which offer has the longest lease or which offer provides the most information that the specific client needs for optimal operation.
  • The non-chosen servers would notice the explicit DHCPREQUEST packet and go on about their business.
  • The chosen server would return a DHCPACK that tells the client the lease is finalized
  • Once the client receives a DHCPACK, then all ownership and maintenance of the lease is the responsibility of the client
  • Once the client has the lease, it must be renewed prior to the lease expiration through another DHCPREQUEST message.
  • If a client finishes using a lease prior to its expiration date, the client is supposed to send a DHCPRELEASE message to the server so that the lease can be made available to other nodes.
  • If the server doesn’t hear from the client by the end of the lease, it marks the lease as non-renewed, and makes it available for other clients to use.


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