InetAddress Class in Java

The InetAddress class represents an IP address, either IPv4 (such as 192.168.1.20) or IPv6 (such as ::1). It is also Java's way of asking DNS to turn a host name into an address. InetAddress has no public constructor: you get objects from static factory methods. An InetAddress object is immutable. In this lesson, you will learn how to create addresses, look up host names, test what kind of address you have, and combine an address with a port.

Methods

Inet4Address and Inet6Address are its two subclasses; the factory methods return the right one.

Example: Creating and Inspecting Addresses

import java.net.Inet4Address;
import java.net.Inet6Address;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.UnknownHostException;
import java.util.Arrays;
 
public class Addr {
    public static void main(String[] args) throws Exception {
        InetAddress loop = InetAddress.getLoopbackAddress();
        System.out.println(loop.getHostAddress()
            + " loopback? " + loop.isLoopbackAddress());
 
        InetAddress home = InetAddress.getByName("192.168.1.20");
        System.out.println(home.getHostAddress()
            + " private? " + home.isSiteLocalAddress());
        System.out.println(Arrays.toString(home.getAddress()));
        System.out.println("IPv4? " + (home instanceof Inet4Address));
 
        InetAddress raw = InetAddress.getByAddress(
            new byte[] {(byte) 203, 0, 113, 7});
        System.out.println(raw.getHostAddress()
            + " private? " + raw.isSiteLocalAddress());
 
        InetAddress v6 = InetAddress.getByName("::1");
        System.out.println(v6.getHostAddress() + " IPv6? "
            + (v6 instanceof Inet6Address)
            + " loopback? " + v6.isLoopbackAddress());
 
        InetSocketAddress endpoint =
            new InetSocketAddress("127.0.0.1", 8080);
        System.out.println(endpoint.getAddress().getHostAddress()
            + " port " + endpoint.getPort());
 
        System.out.println(InetAddress.ofLiteral("10.0.0.5").getHostAddress());
        try {
            InetAddress.getByName("no-such-host.invalid");
        } catch (UnknownHostException e) {
            System.out.println("UnknownHostException for a name"
                + " that doesn't exist");
        }
        try {
            InetAddress.ofLiteral("example.com");
        } catch (IllegalArgumentException e) {
            System.out.println("ofLiteral: " + e.getMessage());
        }
    }
}

127.0.0.1 loopback? true
192.168.1.20 private? true
[-64, -88, 1, 20]
IPv4? true
203.0.113.7 private? false
0:0:0:0:0:0:0:1 IPv6? true loopback? true
127.0.0.1 port 8080
10.0.0.5
UnknownHostException for a name that doesn't exist
ofLiteral: Invalid IP address literal: example.com

Explanation:

• Loopback (127.0.0.1, or ::1 in IPv6) always means "this computer".
• getByName("192.168.1.20") is given an address, not a name, so it only parses the text. isSiteLocalAddress() is true because 192.168.x.x is a private range used inside homes and offices; so are 10.x.x.x and 172.16.x.x to 172.31.x.x.
• The bytes print as -64, -88 because Java's byte is signed: 192 is stored as 192 - 256 = -64. That is also why building an address needs the cast (byte) 203.
• InetSocketAddress is an address plus a port: one complete endpoint. Sockets and servers accept it directly.
• getByName with a name that doesn't exist throws the checked UnknownHostException.
• ofLiteral refuses anything that isn't an address, so it can never trigger a slow or unexpected DNS lookup. Use it when the text is supposed to be an IP address.

Looking Up Real Host Names

These calls need a network connection, and the answers depend on where and when you run them:

InetAddress site = InetAddress.getByName("www.poolstring.com");
System.out.println(site.getHostAddress());
 
for (InetAddress a : InetAddress.getAllByName("www.poolstring.com")) {
    System.out.println(a);
}
 
System.out.println(InetAddress.getLocalHost());

A lookup blocks until DNS answers or gives up, which can take seconds. Java caches successful lookups (by default for 30 seconds), so repeating one is fast.

IPv4 and IPv6

Code written against InetAddress works with both, so you rarely need the subclasses.

Common Mistakes

Calling new InetAddress(...). There is no public constructor.

Passing user input to getByName when you expect an IP address. It will do a DNS lookup on whatever text it gets. Use ofLiteral.

Forgetting that bytes are signed. Values above 127 need a cast, and print as negative numbers.

Using getLocalHost() to find "my IP address". A computer can have several addresses; this returns only the one for its host name.

Interview Corner

Practice

1. Easy: Print whether each of 10.1.2.3, 8.8.8.8 and 172.20.0.1 is a private (site-local) address.
2. Harder: Write a method that takes a string and returns "IPv4", "IPv6" or "not an address", using InetAddress.ofLiteral.

Key Takeaways