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Use InetAddress's getByName(String) class method.

     The InetAddress class's getByName method translates from host names to InetAddress instances. By delivering InetAddress instances with a method rather than a constructor, the InetAddress class gains in flexibility and efficiency. It can create new InetAddress instances if it needs to, but it also has the option of returning a reference to an InetAddress instance it has already created and saved. This kind of caching is hidden from you as an implementation detail, but it is important for efficiency and performance.
     getByName is a class method, which means you invoke it on the InetAddress class. You provide a String argument that represents either the symbolic name of the host or its numeric address. This method can throw an UnknownHostException, so your code must either handle or declare that exception. The following code fragment illustrates:

I     netAddress address1;
     InetAddress address2;
     try {
         address1 = InetAddress.getByName("java.sun.com");
         address2 = InetAddress.getByName("192.9.9.100");
     } catch (UnknownHostException e) { /* ... */ }

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Yes; use InetAddress's getLocalHost method.

InetAddress's getLocalHost method returns the address of the local host, if the method can find it; if not, the method throws an UnknownHostException. The following code fragment obtains an InetAddress instance representing the local host, from which it then gets the local host's name and IP address:

     InetAddress localHost = null;
     String localHostName;
     byte[] byteAddress;
     try {
         localHost = InetAddress.getLocalHost();
     } catch (UnknownHostException e) { /* ... */ }
     localHostName = localHost.getHostName();
     byteAddress = localHost.getAddress();

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A bug in the JDK 1.0.2 implementation prevents this from working, but in the JDK 1.1 you can use InetAddress's getByName method (using a number.number. number.number address) even if no DNS entry exists for the address.

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Like this:

URL url = new URL("http://www.my_domain.com/my_page.html");
URL anchor = new URL(url, "#section2");
this.getAppletContext().showDocument(anchor);

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Let's start by noting that this is more troublesome than it might seem at first, and that GET is preferred. For an untrusted applet, the CGI script can only be on the server that served the applet. POSTing to
the server involves sending key/value pairs, not just a message. Also values must be encoded. To send "words to the server" you might try:

    StringBuffer sb = new StringBuffer();
    String str="words to the server";

    sb.append("message=");
    sb.append(java.net.URLEncoder.encode(str));
    URLConnection cn = url.openConnection();

    // set properties
    cn.setDoOutput(true);
    cn.setUseCaches(false);
    cn.setAllowUserInteraction(false);
    cn.setRequestProperty("content-type","application/x-www-form-urlencoded");

     // send parameters
     PrintWriter out = new PrintWriter(cn.getOutputStream());
     out.print(sb.toString());
     out.close();

     // read stuff
    BufferedReader in = new BufferedReader(
    new InputStreamReader(
    cn.getInputStream()));

    sb = new StringBuffer();
    String inputLine;

    while ((inputLine = in.readLine()) != null){
    sb.append(inputLine);
    sb.append("\n");
  }
  in.close();


  uresp = new URL(getDocumentBase(),"respond.html");
  getAppletContext().showDocument(uresp); }

The CGI has to write its output to respond.html so that it can be displayed by the browser. But it may still fail, because respond.html could be overwritten by a subsequent request to the same CGI before the
results of the first POST are read back.
To get an acceptable solution takes quite a lot of effort. In general you should prefer GET to POST for CGI access from Java. As it says on the Javaworld page, the answers to the question are really: you can't, don't POST (use GET), use a bean, or cheat.

Note, if you request a URL via the URLConnection/HttpURLConnection, the server sets the content type, and your applet can use URLConnection.getContentType() to get the type. Alternatively, use setRequestProperty to set it, like this:

url = new URL(cgiUrl);
urlc = url.openConnection();
urlc.setRequestProperty(
"Content-type",
"application/x-www-form-urlencoded");

Other sites:
There's a pretty good explanation at
http://www.javaworld.com/javaworld/javatips/jw-javatip41.html

See also the Marty Hall book Core Web Programming (Prentice Hall ISBN: 0-13-625666-X). It has comprehensive coverage of writing Java programs that interface to CGI scripts. Better still, use servlets, write everything in Java, and discard your CGI code. 

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CGI (the Common Gateway Interface for web servers) is an API for writing programs that use the web as its user interface. By far, the most popular language for this task is Perl, because of its powerful  text handling capabilities, and excellent resources available for  making the jobs of CGI programmers easier. CGI programs can be written in any language, including Java. Unfortunately, the interface between the web server and the CGI program  uses environment variables extensively. Use of environment variables  has always been deprecated in Java, because of portability issues (not all systems have environment variables). The way to get around this is to write a "wrapper" for the Java program in a language that supports environment variables, and can then invoke the Java program with the  appropriate environment data passed in as Java properties. Because the Java runtime environment is not a lightweight process, it might take a moment for the CGI program to get started before anything    happens. This is particularly true on operating systems, like NT, that  have a large overhead to spawning new processes.  In preference to using Java for CGI on the server, you might consider using the Java servlet API in Netscape's Enterprise Server. This allows  you to develop server-side programs in Java without suffering the same performance restrictions and other limitations of the CGI API.  Other sites:

     See http://search.netscape.com/comprod/server_central/query/eval_guide/enterprise/advantage.html for more details.

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In Java 1.1 (earlier releases were buggy) use:

String host = InetAddress.getByName("211.10.2.119").getHostName();

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This is a platform difference that arises out of different semantics in the underlying network libraries, and is [said to be, but subject to confirmation] fixed in JDK 1.1. On Solaris and Windows NT, the IP address string only works for IP addresses that have an associated hostname. On Linux and Windows 95, the IP address string works in all cases.
When InetAddress is instantiated with an IP address, a reverse DNS lookup is done. If the IP address is not associated with a valid hostname, the instantiation will fail. This is part of anti DNS-spoofing, and in JDK 1.1 works because the reverse lookup will not occur until the hostname is asked for. So in JDK 1.1,

InetAddress in = InetAddress.getByName("155.152.5.1");

[Note: this info is still to be confirmed. Net gurus?]
Other sites: Microsoft has several network-related patches at its site http://www.microsoft.com/ 

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The InetAddress has a static method called getLocalHost() which will return the current address of the local machine. You can then use the getHostAddress() method to get the IP address.

InetAddress local = InetAddress.getLocalHost();

// Print address
System.out.println ("Local IP : " + local.getHostAddress());

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There is an SSL class available from http://www.phaos.com/ Netscape says they'll include an SSL class in Netscape 4.0. JavaSoft has one too, which ships with HotJava and the JavaWebServer. 

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Ping requires ICMP packets. These packets can only be created via a socket of the SOCK_RAW type. Currently, Java only allows SOCK_STREAM (TCP) and SOCK_DGRAM (UDP) sockets. It seems unlikely that this will be added very soon, since many Unix versions only allow SOCK_RAW sockets to be created by root, and winsock does not address ICMP packets (win32 includes an unsupported and undocumented ICMP.DLL). 

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Returning the fully qualified hostname is not guarenteed. Currently, whether this series of calls does or not is platform dependant. On Solaris, it will return only the unqualified host name. On Windows, it may return the fully qualified name, or it may not. It will depend on the version of Winsock in use. If you are using the Microsoft Winsock, you will get the fully qualified name. The 1.1 FCS JDK should have a fix for the Solaris part of this problem, anyway. 

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Unfortunately due to an unintended side effect (i.e. a bug) in Java's caching of IP addresses and hostnames, Java 1.0 can't convert numeric IP addresses into hostnames. However this is straightforward in Java 1.1. For example,
     String hostname = InetAddress.getByName("199.1.32.90").getHostName()

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The InetAddress class is able to resolve IP addresses for you. Obtain an instance of InetAddress for the machine, and call the getHostAddress() method, which returns a string in the xxx.xxx.xxx.xxx address form.

InetAddress inet = InetAddress.getByName("www.davidreilly.com");
System.out.println ("IP : " + inet.getHostAddress());