Structure and allocation of DHCP protocol

Next we will mainly explain the content about DHCP. In terms of network usage, we have talked about the TCP/IP protocol, including many protocols. Here, we will first emphasize some basic knowledge about DHCP. Next, let's take a look at the main content.

DHCP protocol Concept

Dynamic Host Configuration Protocol (DHCP) is a LAN network Protocol that uses UDP for two purposes: an IP address is automatically assigned to an internal network or network service provider to the user as a central management means for all computers.

DHCP protocol structure 

The DHCP Protocol is short for the Dynamic Host Configuration Protocol (Dynamic Host Configuration Protocol), and its predecessor is BOOTP. BOOTP was originally used on a network without a disk host connection: the network host uses boot rom instead of the disk to start and connect to the network. BOOTP can automatically set the TCP/IP environment for those hosts. However, BOOTP has one disadvantage: You must obtain the hardware address of the client before setting it, and the corresponding IP address is static. In other words, BOOTP lacks "dynamic". In a limited IP resource environment, a pair of BOOTP will cause a considerable waste.

DHCP can be said to be an enhanced version of BOOTP. It is divided into two parts: one is the server, and the other is the client. All IP network settings are centrally managed by the DHCP server and are responsible for handling DHCP requirements of the client. The client uses IP environment data allocated from the server. Compared with BOOTP, DHCP uses the "lease" concept to effectively and dynamically allocate TCP/IP settings of the client. In addition, DHCP takes full care of BOOTPClient's needs as compatibility considerations. The allocation form of DHCP protocol first requires at least one DHCP server to work on the network. It listens to DHCP requests on the network and negotiates the TCP/IP setting environment with the client. It provides three IP address locating methods:

Address Allocation Method

The Network Administrator binds a fixed IP address to a few specific hosts and the address does not expire.

Automatic Allocation

Once the DHCP Client is successfully rented from the DHCP server to an IP address for the first time, it will always be used.

Dynamic Allocation

After DHCP is rented from the DHCP server for the first time to an IP address, it is not permanently used. As long as the lease term expires, the client must release the IP address (release, for other workstations. Of course, the client can give priority to the update lease or lease other IP addresses than other hosts.

Dynamic Allocation is obviously more flexible than automatic allocation, especially when your actual IP address is insufficient. For example, if you are an ISP, you can only provide 200 IP addresses for the customer, however, this does not mean that you can have a maximum of 200 customers. You must know that your customers cannot access the Internet at the same time. Apart from their different behavior habits, they may also be limited by telephone lines. In this way, you can lease these 200 addresses to the connected customers in turn. This is also the reason why the IP address is different each time you view the IP address (unless you apply for a fixed IP address, usually the ISP can meet this requirement, this may be charged separately ). Of course, ISP does not necessarily use DHCP to allocate addresses, but this concept is the same as IP Pool.

In addition to dynamically setting IP addresses, the DHCP protocol can also retain some IP addresses for use by machines with special purposes. It can assign fixed IP addresses based on hardware addresses, this gives you a larger design space. In addition, DHCP can also help clients specify projects such as router, netmask, DNS Server, and WINS Server, there is almost no need to set any IP environment.