Dynamic address allocation DHCP, IP address management method and allocation principles, learn about it in one minute

1. Introduction to DHCP

DHCP (Dynamic Host Configuration Protocol) is used to dynamically allocate network configuration parameters such as IP addresses to network devices.

DHCP uses a client/server communication model, where the client submits an application to the server to request the allocation of network configuration parameters, and the server returns configuration information such as the IP address allocated to the client to achieve dynamic configuration of information such as the IP address.

In a typical DHCP application, there is usually a DHCP server and multiple clients (such as PCs and laptops).

2. DHCP IP address allocation

1. IP address allocation strategy

In response to different needs of clients, DHCP provides three IP address allocation strategies:

• Manual address allocation: The administrator statically binds a fixed IP address to a few specific clients (such as WWW servers, etc.). The configured fixed IP address is allocated to the client through DHCP.
• Automatic address allocation: DHCP allocates IP addresses with an unlimited lease period to clients.
• Dynamically allocated addresses: DHCP allocates IP addresses with a certain validity period to clients. After the validity period expires, the client needs to reapply for an address. Most clients get this type of dynamically allocated address.

2. IP address acquisition process

The DHCP client obtains an IP address from the DHCP server in four stages:

• The discovery phase is when the DHCP client searches for a DHCP server. The client sends a DHCP-DISCOVER message in broadcast mode.
• The provision phase is the phase in which the DHCP server provides IP addresses. After receiving the DHCP-DISCOVER message from the client, the DHCP server selects an IP address according to the priority of IP address allocation and sends it to the client together with other parameters through a DHCP-OFFER message.
• The selection phase is the phase in which the DHCP client selects an IP address. If multiple DHCP servers send DHCP-OFFER messages to the client, the client only accepts the first DHCP-OFFER message received, and then broadcasts a DHCP-REQUEST message, which contains the IP address assigned by the DHCP server in the DHCP-OFFER message.
• The confirmation phase is the phase in which the DHCP server confirms the IP address. After the DHCP server receives the DHCP-REQUEST message from the DHCP client, only the server selected by the DHCP client will perform the following operations: if it confirms that the address will be allocated to the client, it will return a DHCP-ACK message; otherwise, it will return a DHCP-NAK message, indicating that the address cannot be allocated to the client.

After receiving the DHCP-ACK message from the server, the client will broadcast a free ARP message to detect whether there is a host using the IP address assigned by the server. If no response is received within the specified time, the client will use this address. Otherwise, the client will send a DHCP-DECLINE message to the DHCP server and re-apply for an IP address.

If there are multiple DHCP servers in the network, in addition to the server selected by the DHCP client, the IP addresses not allocated this time in other DHCP servers can still be allocated to other clients.

3. IP address lease renewal

The IP address assigned by the DHCP server to the client has a certain lease period (except for the automatically assigned IP address), which is called a lease. When the lease expires, the server will reclaim the IP address. If the DHCP client wants to continue using the address, it needs to apply for an extension of the IP address lease.

When the IP address lease of the DHCP client reaches about half of the time, the DHCP client will unicast a DHCP-REQUEST message to the DHCP server that assigned the IP address to it to renew the IP lease. If the client can continue to use this IP address, the DHCP server responds with a DHCP-ACK message to inform the DHCP client that a new IP lease has been obtained; if this IP address can no longer be assigned to the client, the DHCP server responds with a DHCP-NAK message to inform the DHCP client that it cannot obtain a new lease.

If the renewal operation fails at about half of the lease time, the DHCP client will broadcast a DHCP-REQUEST message to renew the lease when the lease period reaches 7/8.