How do I work with InetAddress to resolve IP addresses in Java?

In Java, InetAddress is part of the java.net package and is used to handle IP addresses, both IPv4 and IPv6. It provides methods to resolve hostnames to IP addresses and vice versa, as well as to work with local and remote host information. Here’s a detailed explanation of how to work with InetAddress to resolve IP addresses:

Basic Workflow with `InetAddress`

Import the Necessary Package

import java.net.InetAddress;
import java.net.UnknownHostException;

Resolve a Hostname to an IP Address Use InetAddress.getByName(String hostname) to find the IP address of a given hostname:

try {
   InetAddress address = InetAddress.getByName("www.example.com");
   System.out.println("Host Name: " + address.getHostName());
   System.out.println("IP Address: " + address.getHostAddress());
} catch (UnknownHostException e) {
   System.err.println("Host not found: " + e.getMessage());
}

Get the Local Host Address Use InetAddress.getLocalHost() to retrieve the IP address of your local machine:

try {
   InetAddress localHost = InetAddress.getLocalHost();
   System.out.println("Local Host Name: " + localHost.getHostName());
   System.out.println("Local IP Address: " + localHost.getHostAddress());
} catch (UnknownHostException e) {
   System.err.println("Unable to get local host address: " + e.getMessage());
}

Resolve All IP Addresses for a Hostname Some hostnames might resolve to multiple IP addresses (e.g., websites using load balancing). Use InetAddress.getAllByName(String hostname) to get all associated IPs:

try {
   InetAddress[] addresses = InetAddress.getAllByName("www.google.com");
   for (InetAddress address : addresses) {
       System.out.println("IP Address: " + address.getHostAddress());
   }
} catch (UnknownHostException e) {
   System.err.println("Host not found: " + e.getMessage());
}

Work with the Loopback Address You can retrieve and work with the default loopback address (127.0.0.1 or ::1):

InetAddress loopback = InetAddress.getLoopbackAddress();
System.out.println("Loopback Address: " + loopback.getHostAddress());

Check Reachability Use InetAddress.isReachable(int timeout) to check if an address is reachable (via ICMP ping-like mechanism):

try {
   InetAddress address = InetAddress.getByName("www.example.com");
   if (address.isReachable(5000)) { // Timeout in milliseconds
       System.out.println(address + " is reachable.");
   } else {
       System.out.println(address + " is not reachable.");
   }
} catch (Exception e) {
   System.err.println("Error checking reachability: " + e.getMessage());
}

Create an InetAddress from an IP String If you already have an IP address in string form and want to create an InetAddress object:

try {
   InetAddress address = InetAddress.getByName("192.168.0.1");
   System.out.println("Host Name: " + address.getHostName());
   System.out.println("IP Address: " + address.getHostAddress());
} catch (UnknownHostException e) {
   System.err.println("Invalid IP address: " + e.getMessage());
}

Useful Methods in `InetAddress`

Method	Description
`getByName(String host)`	Returns `InetAddress` for the given hostname or IP address.
`getAllByName(String host)`	Returns all `InetAddress` objects for the given hostname.
`getLocalHost()`	Fetches the local machine’s `InetAddress`.
`getLoopbackAddress()`	Returns the loopback address (e.g., `127.0.0.1`).
`getHostName()`	Returns the hostname for the IP address.
`getHostAddress()`	Gets the IP address as a string.
`isReachable(int timeout)`	Checks if the address is reachable within a given timeout.
`equals(Object obj)`	Compares two `InetAddress` objects.
`hashCode()`	Generates the hash code for the `InetAddress` instance.

Common Use-Case Examples

Example 1: Resolving a Hostname

try {
    InetAddress address = InetAddress.getByName("www.example.com");
    System.out.println("IP Address: " + address.getHostAddress());
} catch (UnknownHostException e) {
    System.err.println("Unable to resolve host: " + e.getMessage());
}

Example 2: Listing All IPs for a Domain

try {
    InetAddress[] addresses = InetAddress.getAllByName("www.google.com");
    for (InetAddress addr : addresses) {
        System.out.println(addr);
    }
} catch (UnknownHostException e) {
    System.err.println("Unable to resolve host: " + e.getMessage());
}

Example 3: Checking Local Host Information

try {
    InetAddress localHost = InetAddress.getLocalHost();
    System.out.println("HostName: " + localHost.getHostName());
    System.out.println("IP Address: " + localHost.getHostAddress());
} catch (UnknownHostException e) {
    System.err.println("Error: " + e.getMessage());
}

Notes:

In modern applications, DNS caching and network configurations can affect resolution.
InetAddress.getByName internally resolves the hostname using DNS.
For non-blocking or asynchronous networking, consider Java’s java.nio package.

With these explanations and examples, you should be able to handle IP address resolution reliably using InetAddress.

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Basic Workflow with InetAddress

Useful Methods in InetAddress