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Can you please explain how a DHCP server allocates IP addresses dynamically?

Learn how DHCP works in this Ask the Expert response from our networking fundamentals expert, Chris Partsenidis.

Can you please explain how a DHCP server allocates IP address dynamically to systems on a network and how these systems are connected to the server?

Dynamic Host Configuration Protocol (DHCP) is a service most networks, and especially large ones, use. The DHCP

service allows automatic configuration of all network hosts which support DHCP.

On the DHCP server, we usually configure the network parameters, which include IP address range assigned to the hosts, Gateway (or router), DNS server(s) and a few more. These parameters are then passed down to the host as if you manually configured it.

The process that describes DHCP is what I like to call "DORA" and here's the explanation:

  1. Discover
  2. Offer
  3. Request
  4. Accept

Let's now take each one and briefly explain what happens:

  1. Discover: The host will initially send a broadcast in an attempt to discover a DHCP server on the network.
  2. Offer: The DHCP server will 'see' the workstation looking for the DHCP service and respond with an 'offer', which is an IP address.
  3. Request: The client will receive the 'offer' and, in most cases, will accept it. This means it sends an 'official request' for the same IP address offered previously by the server.
  4. Accept: The DHCP server will complete the transaction by sending an 'accept' message and marking the particular IP address for the specific host.

I hope the above covers you completely!

This was first published in August 2004

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