DHCP (Dynamic Host Configuration Protocol)


What is DHCP?

In a network all the Layer 3 devices need to have an IP address. We can give these IP addresses manually one by one or dynamically and automatically with IP address allocation mechanism, DHCP (Dynamic Host Configuration Protocol. Here, beside IP addresses, we can configure the allocation of other IP related configurations with DHCP. In this lesson, we will learn What is DHCP? In another lesson, we will see How to Configure DHCP With Packet Tracer?



DHCP IP Allocation Mechanisms


DHCP (Dynamic Host Configuration Protocol) has three DHCP IP Allocation Mechanisms. These are:

  • Dynamic
  • Automatic
  • Manual

The first one is Dynamic IP Allocation. Dynamic IP Allocation, allocates IP addresses from a predefined Pool and can be cancelled.

Secondly, Automatic IP Allocation. In Automatic IP Allocation, allocates IP addresses from a predefined Pool too, but this time can not be cancelled.

Thirdly, Manuel IP Allocation. In Manuel IP Allocation, allocates IP addresses according to request of host and this is a range out of the pool.

If, DHCP assigns IP addresses from its predefined IP Pools dynamically, it leases these IP addresses for a short time to the DHCP Client. And DHCP Client uses this IP address until it expires. Before the expiration, DHCP Client request renewal or another IP address.


You can watch DHCP Everview lesson also  on Youtube!



DHCP Devices and Roles


In DHCP (Dynamic Host Configuration Protocol) terminology, there are different devices and roles. These devices and roles are:

  • DHCP Server
  • DHCP Client
  • DHCP Relay Agent


First of all, we do the configurations on DHCP Server. These configurations are, all the configurations like IP Address Pools, Gateway Address etc. It is the mechanims that has the IP pools and provides IP addresses to the other nodes.

Secondly, DHCP Client is the other hosts here. The devices that need IP in a network is the DHCP Client for DHCP Server. To be so, it is also need to be configured to use this DHCP Server.


Thirdly, DHCP Relay Agent is the device stays between the DHCP Client and DHCP Server if these two main DHCP elements are in different networks. Some of the messages of the DHCP are broadcast. So, normally, Broadcast do not pass from one network to an another network. Here, Relay agents converts these broadcast messages to unicast messages.





DHCP Messages


DHCP IP Address Allocation is done via DHCP messages.


With these messages, DHCP Server that will assign IP address, is determined, IP address assignments is done and other IP allocation actions achieved.

What are these DHCP Messages? These DHCP messages are:

  • DHCP Discover
  • DHCP Offer
  • DHCP Request
  • DHCP Ack
  • DHCP Nak
  • DHCP Release
  • DHCP Decline
  • DHCP Inform


The first four of these messages are for a successfull DHCP IP Allocation Operation.

We will talk about DHCP Messages deeply, in the DHCP IP Allocation Operation article.

Below, you can see the picture of this details ;)



Are There Any Other IP Allocation Protocol?


IP Allocation is an important process for networks. And DHCP does IP Allocation duty very efficiently.

There is also another protocol for IP allocation that is very old. This Protocol is BOOTP. The long name of it is BOOTstrap Protocol. Previously, it was used to assign IP addresses to the devices during boot process. But, nowadays, it is rarely used.

Lesson tags: DHCP
Back to: CCNA 200-301 v1.1 > DHCP and DNS

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CCNA 200-301 v1.1