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Network Management

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Check the IP address.

To check the IP address, use the "ifconfig" command. In addition to checking the IP address, the ifconfig command can also be used to change it. On CentOS7, you can also run the "ip" command with the "a" option instead of ifconfig. Currently, ifconfig is being replaced by the ip command, and ifconfig may no longer be available in the future.
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Check the Linux network environment.

Check the Linux network environment. ●Use "ip -a" to check t...
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Check the port status of Linux

The "IP address" is used to identify which computer on the network you are connecting to, and the "port number" is needed to identify which program running on that computer you are accessing. The port number is a 16-bit integer and ranges from 0 to 65535. To check which ports are waiting for a connection in Linux, use the ss command or netstat command. ●well-known ports Port numbers (0-1023) are reserved in advance for well-known services and protocols used in TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).
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Find IP address from hostname

Looking up an IP address from a host name is called "forward DNS lookup". This process can be done by making a query to the name server (DNS) to receive IP address information. ●Use the dig command to find out the IP address. Try to find the IP address of the Yahoo hostname "https://www.yahoo.co.jp/". Check the ANSWER SECTION of the execution result, and you will see the IP address of "https://www.yahoo.co.jp/".
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Look up a mail server by domain name

To find out which host is running as a mail server within a domain, you can check the MX (Mail Exchanger) record set for that domain. In this section we will examine the mailservers for the metro.tokyo.jp domain. From the result of the dig command, you can see that there is a mail server named ml1.tisc.tokyo.jp running in the metro.tokyo.jp domain.
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Configuring the default gateway (router)

●Confirmation of the current default gateway Execute the "route -n" command. "-n" is an option to display the hostname as IP. ●Remove the default gateway Set "del" as the argument of the route command, and set the IP address of the default gateway device (192.168.11.1) that you want to delete after "default gw". ●Setting up a new default gateway Set "add" as the argument of the route command, and set the IP address (192.168.11.2) of the default gateway device you want to set after "default gw".