When an InetAddress instance is created from an IP-address, and getHostName() is called, and the IP-address doesn't have a hostname, then getHostName() will unexpectedly return the IP-address itself as hostname.
This strange gehavior isn't documented: https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/net/InetAddress.html#getHostName() The documentation does mention that if the operation isn't allowed by the security manager, then it will cause this behavior. But even when no security manager is set, if no hostname exists, it will return the IP-address itself. This behavior differs from command line tools like nslookup, which do say that no domain exists. In my opinion, it should either throw an UnknownHostException, return null or return an Optional<String>. The underlying code that is called by getHostName(), is a private interface NameService, which has getHostByAddr(), which does work as expected: it throws an UnknownHostException if the IP-address doesn't have a hostname. But getHostName() swallows this exception, and then returns the IP-address instead, which is wrong behavior. IMO even when a security manager blocks the request, it should never return the IP-address as hostname, but simply re-throw the SecurityException from NameService.getHostByAddr(). You can see the problem causing code here: https://github.com/openjdk/jdk/blob/jdk-17+35/src/java.base/share/classes/java/net/InetAddress.java#L697 JDK 18 has a modified getHostname(), but it still has this unexpected behavior. https://github.com/openjdk/jdk/blob/jdk-18+25/src/java.base/share/classes/java/net/InetAddress.java Here are some examples of people who run into this unexpected behavior: https://stackoverflow.com/questions/1899288/cannot-get-hostname-from-gethostname https://stackoverflow.com/questions/34436938/why-is-javas-inetaddress-gethostname-not-giving-me-the-hosts-name https://stackoverflow.com/questions/24458460/inetaddress-gethostname-returning-ipaddres-on-android
