ネットワークコマンド入門：`ping`、`ip`、`netstat`

Welcome to this lesson on essential Linux networking commands! Understanding these tools is crucial for diagnosing network issues and viewing your system's network configuration.

We'll explore ping for checking connectivity, ip for comprehensive network interface management, and netstat for viewing active connections.

The ping command is a fundamental tool used to test the reachability of a host on an Internet Protocol (IP) network. It sends small data packets to a target and waits for a response.

• Purpose: Check if a remote computer or server is online.
• Measurement: Calculates the Round-Trip Time (RTT) for packets to reach the host and return.
• Analogy: Think of it like a sonar pulse for your network!

Let's try using ping to check connectivity to a well-known website. The command will send continuous pings until you stop it (usually with Ctrl+C).

Observe the output: the time it takes for packets to return and the 'TTL' (Time To Live) value, which indicates how many 'hops' the packet can make before being discarded.

Sometimes you don't need continuous pings. The -c option allows you to specify the number of echo requests to send.

This is useful for a quick check without having to manually stop the command. After sending the specified number of packets, ping will provide a summary.

The ip command is a modern and powerful utility for showing or manipulating routing, network devices, interfaces, and tunnels. It's designed to replace older tools like ifconfig and route.

ip is versatile, with many subcommands for different tasks. We'll look at ip addr for addresses and ip route for routing information.

To view the IP addresses assigned to your network interfaces, you use the ip addr show (or simply ip a) subcommand.

• lo: The loopback interface (127.0.0.1), used for communication within the same machine.
• eth0/wlan0: Your primary wired (Ethernet) or wireless interface.

Look for the inet lines to find your IPv4 addresses.

The ip route show command displays your system's routing table. The routing table tells your computer how to send network traffic to different destinations.

A key entry is the default via route, which indicates your default gateway – the router your computer uses to send traffic to the internet or other networks it doesn't know directly.

netstat (network statistics) is a command-line tool that displays network connections (both incoming and outgoing), routing tables, and a number of network interface statistics.

While ip is often preferred for configuration, netstat remains useful for quickly checking what services are listening on your machine and what connections are established.

To see active network connections and listening ports, you can use netstat with several options:

• -t: Show TCP connections.
• -u: Show UDP connections.
• -l: Show only listening sockets.
• -p: Show the PID/program name owning the socket.
• -n: Show numerical addresses instead of trying to resolve hostnames.

Combining these, netstat -tulpn gives a comprehensive view.

Test your understanding of the networking commands we've covered.

Great job! In this lesson, you've gained an introduction to fundamental Linux networking commands:

• ping: For testing host reachability and measuring round-trip time.
• ip: A powerful, modern tool for viewing and configuring network interfaces and routing tables.
• netstat: Useful for displaying active network connections and listening ports.

Keep practicing these commands to become more comfortable with network diagnostics!