Creating a Subnet for the VPC

Scenarios

A VPC comes with a default subnet. If the default subnet cannot meet your requirements, you can create one.

A subnet is configured with DHCP by default. When an ECS in this subnet starts, the ECS automatically obtains an IP address using DHCP.

Procedure

1. Log in to the management console.

2. Click in the upper left corner and select the desired region and project.
3. Click in the upper left corner and choose Network > Virtual Private Cloud.

   The Virtual Private Cloud page is displayed.

4. In the navigation pane on the left, choose Virtual Private Cloud > Subnets.
5. Click Create Subnet.

   The Create Subnet page is displayed.

6. Set the parameters as prompted.

   Table 1 Parameter descriptions

   Parameter	Description	Example Value
   VPC	The VPC for which you want to create a subnet.	-
   AZ	An AZ is a geographic location with independent power supply and network facilities in a region. AZs are physically isolated, and AZs in the same VPC are interconnected through an internal network. Note the following when you select an AZ: Subnets in a VPC can be in different AZs. For example, a VPC can have a subnet in AZ 1, and another subnet in AZ 3. A cloud resource can be in a different AZ from its subnet. For example, a cloud server in AZ 1 can be in a subnet in AZ 3.	AZ1
   Name	The subnet name. The name can contain a maximum of 64 characters, which may consist of letters, digits, underscores (_), hyphens (-), and periods (.). The name cannot contain spaces.	Subnet
   IPv4 CIDR Block	The CIDR block for the subnet. This value must be within the VPC CIDR block.	192.168.0.0/24
   IPv6 CIDR Block	Specifies whether to set IPv6 CIDR Block to Enable. If you select this option, the system automatically assigns an IPv6 CIDR block to the created subnet. Currently, the IPv6 CIDR block cannot be customized. IPv6 cannot be disabled after the subnet is created.	-
   Associated Route Table	The default route table to which the subnet will be associated. You can change the route table to a custom route table on the Subnets page.	Default
   Gateway	The gateway address of the subnet. This IP address is used to communicate with other subnets.	192.168.0.1
   DNS Server Address	DNS server addresses allow ECSs in a VPC subnet to communicate with each other using private domain names. You can also directly access cloud services through private DNS servers. If you want to use other public DNS servers for resolution, you can change the default DNS server addresses. A maximum of two DNS server IP addresses can be configured. Multiple IP addresses must be separated using commas (,).	100.125.x.x
   NTP Server Address	The IP address of the NTP server. This parameter is optional. You can configure the NTP server IP addresses to be added to the subnet as required. The IP addresses are added in addition to the default NTP server addresses. If you do not specify this parameter, no additional NTP server IP addresses will be added. Enter a maximum of four valid IP addresses, and separate multiple IP addresses with commas. Each IP address must be unique. If you add or change the NTP server addresses of a subnet, you need to renew the DHCP lease for or restart all the ECSs in the subnet to make the change take effect immediately. If the NTP server addresses have been cleared out, restarting the ECSs will not help. You must renew the DHCP lease for all ECSs to make the change take effect immediately.	192.168.2.1
   DHCP Lease Time	The period during which a client can use an IP address automatically assigned by the DHCP server. After the lease time expires, a new IP address will be assigned to the client. Unit: Day or hour If the time period is changed, the new lease time takes effect when the instance (such as an ECS) in the subnet is renewed next time. You can wait for the instance to be renewed automatically or manually modify the lease time. If you want the new lease time to take effect immediately, manually renew the lease or restart the ECS.	-
   Tag	The subnet tag, which consists of a key and value pair. You can add a maximum of 10 tags to each subnet. The tag key and value must meet the requirements listed in Table 2.	Key: subnet_key1 Value: subnet-01
   Description	Supplementary information about the subnet. This parameter is optional. The subnet description can contain a maximum of 255 characters and cannot contain angle brackets (< or >).	-

   Table 2 Subnet tag key and value requirements

   Parameter	Requirements	Example Value
   Key	Cannot be left blank. Must be unique for each subnet. Can contain a maximum of 36 characters. Can contain letters, digits, underscores (_), and hyphens (-).	subnet_key1
   Value	Can contain a maximum of 43 characters. Can contain letters, digits, underscores (_), periods (.), and hyphens (-).	subnet-01

7. Click OK.

Precautions

After a subnet is created, some reserved IP addresses cannot be used. For example, in a subnet with CIDR block 192.168.0.0/24, the following IP addresses are reserved:

• 192.168.0.0: Network ID. This address is the beginning of the private IP address range and will not be assigned to any instance.
• 192.168.0.1: Gateway address.
• 192.168.0.253: Reserved for the system interface. This IP address is used by the VPC for external communication.
• 192.168.0.254: DHCP service address.
• 192.168.0.255: Network broadcast address.

If you configured the default settings under Advanced Settings during subnet creation, the reserved IP addresses may be different from the default ones, but there will still be five of them. The specific addresses depend on your subnet settings.