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getting only url from ip address in java

The following java code will convert an IP Address to its host.
InetAddress addr = InetAddress.getByName("31.13.78.13");
String host = addr.getHostName();
System.out.println(host);
Here host will be printed as "edge-star-shv-01-sit4.facebook.com". The first portion may be the address of a data center of facebook. I only want the "facebook.com" portion. I could not find any built in method in InetAddress library that can sort this out. I can solve the problem using substring method but this is not a good way because I want a generalized approach. Is there any way where I can get "facebook.com" instead of "edge-star-shv-01-sit4.facebook.com" by taking "31.13.78.13" as input?

You can't!
I run multiple websites on one IP adress. So if you would use a script as you discribe it here, what should it return?
The only thing is to get the hostname of the server with the selected IP, this can contain the website name , as in your case, but it doesn't have to. Imagine a server with hostname "example.com" and IP 1.1.1.1, or whatever, on this machine a website in running with url "stackoverflow.com" then you expect when tracing the IP adress to get "stackoverflow.com" but in fact you would get "example.com"

Why is Java's InetAddress getHostName() not giving me the host's name?

I'm trying to get the device's name using its local IP address on the network. Is this how I'm supposed to do it? ex) Arnold-PC, andoid-nnnnnnnnnn
String name = InetAddress.getByName(ip).getHostName();
System.out.println(name);
The above should give me the host's name... but instead gives me the local IP address. - 192.168.2.101
as per the documentation...
public String getHostName ()
Returns the host name corresponding to this IP address. This may or
may not be a fully-qualified name. If the IP address could not be
resolved, the numeric representation is returned instead
Why is it not able to find the host's name?
I don't know much about computer networking... so please excuse my ignorance. :P

check in the command prompt whether you can able to resolve the ip address using nsloookup
if you can't, then your DNS broken
I would like to quote the few lines from java documentation here
getCanonicalHostName() Gets the fully qualified domain name for this
IP address. Best effort method, meaning we may not be able to return
the FQDN depending on the underlying system configuration.
one more trick is to edit host file to get output (not recommended)
Have a look in this answer too
link

From the docs
If this InetAddress was created with a host name, this host name will be remembered and returned; otherwise, a reverse name lookup will be performed and the result will be returned based on the system configured name lookup service. If a lookup of the name service is required, call getCanonicalHostName.
It seems like plain getHostName() won't look up the name if it can't reach out to a DNS server. The DNS server is what will give a name to the host IP address, just like a phone book. Give getCanonicalHostName() a try.

How to figure out an IPv6 address (by name) of a host in my LAN

I have two computers plugged in the same router of a network which I know supports IPv6. Let's call them "PC-A" and "PC-B
I want "PC-A" to figure out "PC-B"s IPv6 address and vice-versa
The first thing I do is
setSystem.setProperty("java.net.preferIPv6Addresses", "true");
If I then say
InetAddress IPAddress = InetAddress.getLocalHost();
I can get my own address which will be in IpV6 format
However, neither of the following two statements gives me "PC-B"s IPv6 address:
Inet6Address IPAddress6 = (Inet6Address)InetAddress.getByName("PC-B");
InetAddress IPAddress = InetAddress.getByName("PC-B");
I also tried to import
import com.lavantech.net.dns.SimpleDNSLookup;
import com.lavantech.net.dns.DNSLookup
The first one I am using as:
SimpleDNSLookup d = new SimpleDNSLookup();
System.out.println(d.getInet6Address("PC-B"));
and the second one as:
DNSLookup dnsLookup = new DNSLookup("PC-B", DNSLookup.QTYPE_AAAA, DNSLookup.QCLASS_IN, 3000, null);
// Get all Address Records.
ResourceRecord[] ansRecords = dnsLookup.getAAAARecords();
System.out.println(ansRecords[0]);
none of which works.
I also tried to use the following
import org.xbill.DNS.*;
int type = Type.AAAA;
Name name = Name.fromString("PC-B");
Lookup lookup = new Lookup(name, type);
lookup.run();
int result = lookup.getResult();
Record[] answers = lookup.getAnswers();
System.out.println(answers[0]);
// (where, for brevity, i am skipping the parts where I check whether result == Lookup.SUCCESSFUL
Note that if I substitute "PC-B" for, say, "ipv6.google.com" I get all the desired results!
Also note that if I just use InetAddress and Type_A wherever applicable in the above approaches, my program returns "PC-B"s IPv4 address without problem.
What am I missing?
Any help is greatly appreciated!

Your question is -unfortunately- a yet unsolved network problem dealing with host discovery on a local subnet (regardless if that subnet has a router or not).
Your desired output is clearly an IPv6 address, but it is unclear what exactly your input is.
Let's focus on PC-B. How exactly do you identify PC-B? It clear that you call it "PC-B", but that name should be configured somewhere before your PC know that that's its name. Where exactly is that configured? Is that the hostname you set on PC-B itself, or is there a domain name server (DNS) where you have given that name? If it is the name in the DNS system, you can indeed query the DNS system for the AAAA record to get the IPv6 address, but you need the fully qualified domain name (FQDN). E.g. "PC-B.yourdomain.com" rather than just "PC-B".
If you know the MAC address of PC-B, you can use the neighbour discovery protocol (NDP) to find out the IP address of PC-B.
There are network protocols that allow PC-A and PC-B to announce their names themselves, once you configured them on the local machines. Such protocols are called "service discovery" protocols, and your options here are (1) multicast DNS (mDNS) and possibly DNS service discovery (DNS-SD) on top of that; or (2) Simple Service Discovery Protocol (SSDP) in UPnP on the other hand. The advantage is that some operating systems already implement this. E.g. if PC-B is a Mac OS X host, all you need to do is query DNS for "pc-b.local" to get the answer. Unfortunately, while implementations of mDNS exist for Linux (Avahi) and Windows (Bonjour), they're not installed by default. A third alternative is to write your own host discovery protocol, and have your hosts run that protocol.
Considerations are which platforms you want to support, if installing third-party software is an option, if the discovery needs to be secure (the above options are not, look into Secure Neighbour Discovery -SEND- if this is a concern), and what input you have in the first place (the hostname "PC-B", or the type of service that runs on PC-B, e.g. _http._tcp for a webserver).

Recommended way to get hostname in Java

Which of the following is the best and most portable way to get the hostname of the current computer in Java?
Runtime.getRuntime().exec("hostname")
vs
InetAddress.getLocalHost().getHostName()

Strictly speaking - you have no choice but calling either hostname(1) or - on Unix gethostname(2). This is the name of your computer. Any attempt to determine the hostname by an IP address like this
InetAddress.getLocalHost().getHostName()
is bound to fail in some circumstances:
The IP address might not resolve into any name. Bad DNS setup, bad system setup or bad provider setup may be the reason for this.
A name in DNS can have many aliases called CNAMEs. These can only be resolved in one direction properly: name to address. The reverse direction is ambiguous. Which one is the "official" name?
A host can have many different IP addresses - and each address can have many different names. Two common cases are: One ethernet port has several "logical" IP addresses or the computer has several ethernet ports. It is configurable whether they share an IP or have different IPs. This is called "multihomed".
One Name in DNS can resolve to several IP Addresses. And not all of those addresses must be located on the same computer! (Usecase: A simple form of load-balancing)
Let's not even start talking about dynamic IP addresses.
Also don't confuse the name of an IP-address with the name of the host (hostname). A metaphor might make it clearer:
There is a large city (server) called "London". Inside the city walls much business happens. The city has several gates (IP addresses). Each gate has a name ("North Gate", "River Gate", "Southampton Gate"...) but the name of the gate is not the name of the city. Also you cannot deduce the name of the city by using the name of a gate - "North Gate" would catch half of the bigger cities and not just one city. However - a stranger (IP packet) walks along the river and asks a local: "I have a strange address: 'Rivergate, second left, third house'. Can you help me?" The local says: "Of course, you are on the right road, simply go ahead and you will arrive at your destination within half an hour."
This illustrates it pretty much I think.
The good news is: The real hostname is usually not necessary. In most cases any name which resolves into an IP address on this host will do. (The stranger might enter the city by Northgate, but helpful locals translate the "2nd left" part.)
In the remaining corner cases you must use the definitive source of this configuration setting - which is the C function gethostname(2). That function is also called by the program hostname.

InetAddress.getLocalHost().getHostName() is the more portable way.
exec("hostname") actually calls out to the operating system to execute the hostname command.
Here are a couple other related answers on SO:
Java current machine name and logged in user?
Get DNS name of local machine as seen by a remote machine
EDIT: You should take a look at A.H.'s answer or Arnout Engelen's answer for details on why this might not work as expected, depending on your situation. As an answer for this person who specifically requested portable, I still think getHostName() is fine, but they bring up some good points that should be considered.

As others have noted, getting the hostname based on DNS resolution is unreliable.
Since this question is unfortunately still relevant in 2018, I'd like to share with you my network-independent solution, with some test runs on different systems.
The following code tries to do the following:
On Windows
Read the COMPUTERNAME environment variable through System.getenv().
Execute hostname.exe and read the response
On Linux
Read the HOSTNAME environment variable through System.getenv()
Execute hostname and read the response
Read /etc/hostname (to do this I'm executing cat since the snippet already contains code to execute and read. Simply reading the file would be better, though).
The code:
public static void main(String[] args) throws IOException {
String os = System.getProperty("os.name").toLowerCase();
if (os.contains("win")) {
System.out.println("Windows computer name through env:\"" + System.getenv("COMPUTERNAME") + "\"");
System.out.println("Windows computer name through exec:\"" + execReadToString("hostname") + "\"");
} else if (os.contains("nix") || os.contains("nux") || os.contains("mac os x")) {
System.out.println("Unix-like computer name through env:\"" + System.getenv("HOSTNAME") + "\"");
System.out.println("Unix-like computer name through exec:\"" + execReadToString("hostname") + "\"");
System.out.println("Unix-like computer name through /etc/hostname:\"" + execReadToString("cat /etc/hostname") + "\"");
}
}
public static String execReadToString(String execCommand) throws IOException {
try (Scanner s = new Scanner(Runtime.getRuntime().exec(execCommand).getInputStream()).useDelimiter("\\A")) {
return s.hasNext() ? s.next() : "";
}
}
Results for different operating systems:
macOS 10.13.2
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
OpenSuse 13.1
Unix-like computer name through env:"machinename"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:""
Ubuntu 14.04 LTS
This one is kinda strange since echo $HOSTNAME returns the correct hostname, but System.getenv("HOSTNAME") does not:
Unix-like computer name through env:"null"
Unix-like computer name through exec:"machinename
"
Unix-like computer name through /etc/hostname:"machinename
"
EDIT: According to legolas108, System.getenv("HOSTNAME") works on Ubuntu 14.04 if you run export HOSTNAME before executing the Java code.
Windows 7
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
Windows 10
Windows computer name through env:"MACHINENAME"
Windows computer name through exec:"machinename
"
The machine names have been replaced but I kept the capitalization and structure. Note the extra newline when executing hostname, you might have to take it into account in some cases.

InetAddress.getLocalHost().getHostName() is better (as explained by Nick), but still not very good
One host can be known under many different hostnames. Usually you'll be looking for the hostname your host has in a specific context.
For example, in a web application, you might be looking for the hostname used by whoever issued the request you're currently handling. How to best find that one depends on which framework you're using for your web application.
In some kind of other internet-facing service, you'll want the hostname your service is available through from the 'outside'. Due to proxies, firewalls etc this might not even be a hostname on the machine your service is installed on - you might try to come up with a reasonable default, but you should definitely make this configurable for whoever installs this.

Although this topic has already been answered there's more to say.
First of all: Clearly we need some definitions here. The InetAddress.getLocalHost().getHostName() gives you the name of the host as seen from a network perspective. The problems with this approach are well documented in the other answers: it often requires a DNS lookup, it's ambiguous if the host has multiple network interfaces and it just plain fails sometimes (see below).
But on any OS there's another name as well. A name of the host that gets defined very early in the boot process, long before the network is initialized. Windows refers to this as computername, Linux calls it kernel hostname and Solaris uses the word nodename. I like best the word computername, so I'll use that word from now on.
Finding the computername
On Linux/Unix the computername is what you get from the C function gethostname(), or hostname command from shell or HOSTNAME environment variable in Bash-like shells.
On Windows the computername is what you get from environment variable COMPUTERNAME or Win32 GetComputerName function.
Java has no way of obtaining what I've defined as 'computername'. Sure, there are workarounds as described in other answers, like for Windows calling System.getenv("COMPUTERNAME"), but on Unix/Linux there's no good workaround without resorting to JNI/JNA or Runtime.exec(). If you don't mind a JNI/JNA solution then there's gethostname4j which is dead simple and very easy to use.
Let's move on with two examples, one from Linux and one from Solaris, which demonstrate how you can easily get into a situation where you cannot obtain the computername using standard Java methods.
Linux example
On a newly created system, where the host during installation has been named as 'chicago', we now change the so-called kernel hostname:
$ hostnamectl --static set-hostname dallas
Now the kernel hostname is 'dallas', as evident from the hostname command:
$ hostname
dallas
But we still have
$ cat /etc/hosts
127.0.0.1 localhost
127.0.0.1 chicago
There's no misconfiguration in this. It just means the host's networked name (or rather the name of the loopback interface) is different from the host's computername.
Now, try executing InetAddress.getLocalHost().getHostName() and it will throw java.net.UnknownHostException. You are basically stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
Solaris example
The example below is based on Solaris 11.3.
The host has deliberately been configured so that the loopback name <> nodename.
In other words we have:
$ svccfg -s system/identity:node listprop config
...
...
config/loopback astring chicago
config/nodename astring dallas
and the contents of /etc/hosts :
:1 chicago localhost
127.0.0.1 chicago localhost loghost
and the result of the hostname command would be:
$ hostname
dallas
Just like in the Linux example a call to InetAddress.getLocalHost().getHostName() will fail with
java.net.UnknownHostException: dallas: dallas: node name or service name not known
Just like the Linux example you are now stuck. There's no way to retrieve neither the value 'dallas' nor the value 'chicago'.
When will you really struggle with this?
Very often you'll find that InetAddress.getLocalHost().getHostName() will indeed return a value which is equal to the computername. So there's no problem (except for the added overhead of name resolution).
The problem arises typically within PaaS environments where there's a difference between computername and the name of the loopback interface. For example people report problems in Amazon EC2.
Bug/RFE reports
A bit of searching reveals this RFE report : link1, link2. However, judging from the comments on that report the issue seems to have been largely misunderstood by the JDK team, so it is unlikely it will be addressed.
I like the comparison in the RFE to other programming languages.

Just one-liner ... cross platform (Windows-Linux-Unix-Mac(Unix)) [Always works, No DNS required]:
String hostname = new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec("hostname").getInputStream()))
.readLine();
You're done !!

Environment variables may also provide a useful means -- COMPUTERNAME on Windows, HOSTNAME on most modern Unix/Linux shells.
See: https://stackoverflow.com/a/17956000/768795
I'm using these as "supplementary" methods to InetAddress.getLocalHost().getHostName(), since as several people point out, that function doesn't work in all environments.
Runtime.getRuntime().exec("hostname") is another possible supplement. At this stage, I haven't used it.
import java.net.InetAddress;
import java.net.UnknownHostException;
// try InetAddress.LocalHost first;
// NOTE -- InetAddress.getLocalHost().getHostName() will not work in certain environments.
try {
String result = InetAddress.getLocalHost().getHostName();
if (StringUtils.isNotEmpty( result))
return result;
} catch (UnknownHostException e) {
// failed; try alternate means.
}
// try environment properties.
//
String host = System.getenv("COMPUTERNAME");
if (host != null)
return host;
host = System.getenv("HOSTNAME");
if (host != null)
return host;
// undetermined.
return null;

The most portable way to get the hostname of the current computer in Java is as follows:
import java.net.InetAddress;
import java.net.UnknownHostException;
public class getHostName {
public static void main(String[] args) throws UnknownHostException {
InetAddress iAddress = InetAddress.getLocalHost();
String hostName = iAddress.getHostName();
//To get the Canonical host name
String canonicalHostName = iAddress.getCanonicalHostName();
System.out.println("HostName:" + hostName);
System.out.println("Canonical Host Name:" + canonicalHostName);
}
}

If you're not against using an external dependency from maven central, I wrote gethostname4j to solve this problem for myself. It just uses JNA to call libc's gethostname function (or gets the ComputerName on Windows) and returns it to you as a string.
https://github.com/mattsheppard/gethostname4j

hostName == null;
Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
{
while (interfaces.hasMoreElements()) {
NetworkInterface nic = interfaces.nextElement();
Enumeration<InetAddress> addresses = nic.getInetAddresses();
while (hostName == null && addresses.hasMoreElements()) {
InetAddress address = addresses.nextElement();
if (!address.isLoopbackAddress()) {
hostName = address.getHostName();
}
}
}
}

Building off of Dan Ortega's answer, I created a generic executeCommand(String) method that takes a command as a paramater.
import java.io.*;
public class SystemUtil {
public static void main(String[] args) throws IOException {
System.out.println(retrieveHostName());
}
public static String retrieveHostName() throws IOException {
return executeCommand("hostname");
}
private static String executeCommand(String command) throws IOException {
return new BufferedReader(
new InputStreamReader(Runtime.getRuntime().exec(command).getInputStream()))
.readLine();
}
}

InetAddress.getLocalHost().getHostName() is the best way out of the two as this is the best abstraction at the developer level.

After Port Forwarding, how to get my external IP in Java?

I set up a static IP and did port forwarding on my notebook, and now I have a static IP address, but it's relatively static, every time I re-start the machine, I get another address, and since I have a "static" IP I can now do Paypal IPN messaging. But how can I get this static IP from my Java program ? One way I can think of is to visit : http://portforward.com/ and on that page it tells me what my external IP is, so I can extract it with Java code, is there any other way that I can do in my Java code to get this info ?

The best solution is probably dynamic DNS. Essentially, you run a program on your computer (or router) that notifies a DNS server when your IP changes. Then, you can just tell PayPal the domain name.

There is a public service you can call with your script to retrieve your external IP address. Bear in mind they have changed the link location once. If you control your own server, you should probably write your own PHP script to simply return the address of the caller to the script.
http://www.whatismyip.com/faq/automation.asp - follow the developers link they provide
import java.net.*;
import java.io.*;
URL myExternalIP = new URL("PUT THE LINK HERE");
BufferedReader in = new BufferedReader(new InputStreamReader(
myExternalIP.openStream()));
String ip = in.readLine(); //you get the IP as a String
System.out.println(ip);
Your own server script is a simple one-liner. Create a file called whatismyip.php and here's the content.
<? echo $_SERVER['REMOTE_ADDR']?>

I see three ways of doing this:
As you discovered, querying an external server what IP you're apparently connecting from. This is simple but you require such a service to be available and, usually, that no transparent proxy messes with your results.
IGD, a sub-protocol of UPnP can give you the result very easily if your port forwarding devices supports it (many do). Google "IGD JAVA" for libraries and samples.
Register for a dynamic DNS service and then lookup your own DNS name.

You can use the NetworkInterface class:
Enumeration<NetworkInterface> ifcs = NetworkInterface.getNetworkInterfaces();
while(ifcs.hasMoreElements()){
NetworkInterface ifc = ifcs.nextElement();
System.out.println("Adresses of: "+ifc.getDisplayName());
Enumeration<InetAddress> adresses = ifc.getInetAddresses();
while(adresses.hasMoreElements()){
System.out.println(adresses.nextElement().getHostAddress());
}
}
This snippet will show you all of the interfaces and the IPs bound to them. You will need to look through the list to find the appropriate interface (see also NetworkInterface.getByName(String name)) And then look at the addresses for that interface. Once you have found the appropriate InetAdress you can use that to get the string or byte representation.