What is DHCP (Dynamic Host Configuration Protocol)?

What is DHCP (Dynamic Host Configuration Protocol)?

When you connect to a network, you usually have an IP address assigned automatically by the router. This process is necessary for devices to be able to communicate with devices on other networks. The switch service that assigns IP addresses is called Dynamic Host Configuration Protocol or DHCP.

DHCP works in a client-server model where a client broadcasts an IP address request to the local network and the DHCP server responds by assigning an IP address. Typically this happens as soon as the device connects to a network so it can start communicating as soon as possible.

When IP addresses are given, they are leased only for a certain period of time, then the DHCP server retrieves the assigned IP address. To avoid this, hosts will start renewing their current IP address allocation halfway through the lease term. Generally, if a lease expires and a new IP address is assigned to a host, a new one is randomly assigned, not the same as the previous one.

In-home and other small networks, the DHCP server is typically integrated into the router because this reduces the number of devices and reduces setup complexity. For large corporate networks, typically a single DHCP server will be used for multiple networks such as the entire building. In this case, DHCP relay agents are used in each interconnected network to forward DHCP traffic from the hosts to the DHCP server and forward the response back to the original host.

Typically, a DHCP server offers connected hosts a random IP address from a configured IP address pool, but it is also possible to manually assign an IP address to specific devices if required.

This is useful if there is some type of server on a network such as a NAS drive or a local webserver. By manually assigning an IP address to these servers, they are always in the same place and you don't have to worry about the IP addresses changing and causing problems.

DHCP Address Lease Stages:

The lease transaction between client and server consists of 4 steps;

1. DHCP Discover
2. DHCP Offer
3. DHCP Request
4. DHCP Acknowledgment

If the client dynamically expects an IP when added to the environment, it does Discover for a DHCP server that can assign IP to it. The broadcast package that the client sends to the environment is called the DHCP Discover package. This process continues four times and if it cannot reach the server that will assign IP, it defines an address from the APIPA class.

If there is a DHCP server in the environment, it sends an Offer to the client. During this process, the server has reserved an IP for the client.

The client accepts the first offer returned to him and sends a Request package as feedback against this offer. At this stage, other servers sending offers to the client withdraw their offers.

In the last stage, the server confirms the previously reserved IP address for the client and sends the Acknowledgment package. Communication is always broadcast because the client does not have an approved IP address.

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