java/8 : java/net/package-info.java

java.net
https://openjdk.java.net/
GPLv2 + Classpath Exception
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Provides the classes for implementing networking applications.
 The java.net package can be roughly divided in two sections:

    A Low Level API, which deals with the
              following abstractions:
    
      Addresses, which are networking identifiers,
             like IP addresses.
      Sockets, which are basic bidirectional data communication
             mechanisms.
      Interfaces, which describe network interfaces. 
    
     A High Level API, which deals with the following
         abstractions:
    
      URIs, which represent
              Universal Resource Identifiers.
      URLs, which represent
              Universal Resource Locators.
      Connections, which represents connections to the resource
              pointed to by URLs.
      

Addresses
Addresses are used throughout the java.net APIs as either host
   identifiers, or socket endpoint identifiers.
The InetAddress class is the abstraction representing an IP (Internet Protocol) address. It has two subclasses: 

      Inet4Address for IPv4 addresses.
      Inet6Address for IPv6 addresses.

But, in most cases, there is no need to deal directly with the subclasses,
   as the InetAddress abstraction should cover most of the needed
   functionality.
About IPv6
Not all systems have support for the IPv6 protocol, and while the Java networking stack will attempt to detect it and use it transparently when available, it is also possible to disable its use with a system property. In the case where IPv6 is not available, or explicitly disabled, Inet6Address are not valid arguments for most networking operations any more. While methods like InetAddress.getByName are guaranteed not to return an Inet6Address when looking up host names, it is possible, by passing literals, to create such an object. In which case, most methods, when called with an Inet6Address will throw an Exception.
Sockets
Sockets are means to establish a communication link between machines over
   the network. The java.net package provides 4 kinds of Sockets:

      Socket is a TCP client API, and will typically be used to 
           connect to a remote host.
      ServerSocket is a TCP server API, and will typically accept connections from client sockets.
      DatagramSocket is a UDP endpoint API and is used to send and receive datagram packets.
      MulticastSocket is a subclass of DatagramSocket used when dealing with multicast groups.

Sending and receiving with TCP sockets is done through InputStreams and OutputStreams which can be obtained via the Socket.getInputStream and Socket.getOutputStream methods.
Interfaces
The NetworkInterface class provides APIs to browse and query all the networking interfaces (e.g. ethernet connection or PPP endpoint) of the local machine. It is through that class that you can check if any of the local interfaces is configured to support IPv6.
Note, all conforming implementations must support at least one NetworkInterface object, which must either be connected to a network, or be a "loopback" interface that can only communicate with entities on the same machine.
High level API
A number of classes in the java.net package do provide for a much higher
   level of abstraction and allow for easy access to resources on the
   network. The classes are:

      URI is the class representing a Universal Resource Identifier, as specified in RFC 2396. As the name indicates, this is just an Identifier and doesn't provide directly the means to access the resource.
      URL is the class representing a Universal Resource Locator, which is both an older concept for URIs and a means to access the resources.
      URLConnection is created from a URL and is the communication link used to access the resource pointed by the URL. This abstract class will delegate most of the work to the underlying protocol handlers like http or https.
      HttpURLConnection is a subclass of URLConnection and provides some additional functionalities specific to the HTTP protocol.

The recommended usage is to use URI to identify resources, then convert it into a URL when it is time to access the resource. From that URL, you can either get the URLConnection for fine control, or get directly the InputStream. 
Here is an example:
URI uri = new URI("http://java.sun.com/");
URL url = uri.toURL();
InputStream in = url.openStream();

Protocol Handlers
 As mentioned, URL and URLConnection rely on protocol handlers which must be present, otherwise an Exception is thrown. This is the major difference with URIs which only identify resources, and therefore don't need to have access to the protocol handler. So, while it is possible to create an URI with any kind of protocol scheme (e.g. myproto://myhost.mydomain/resource/), a similar URL will try to instantiate the handler for the specified protocol; if it doesn't exist an exception will be thrown. By default the protocol handlers are loaded dynamically from the default location. It is, however, possible to add to the search path by setting the java.protocol.handler.pkgs system property. For instance if it is set to myapp.protocols, then the URL code will try, in the case of http, first to load myapp.protocols.http.Handler, then, if this fails, http.Handler from the default location. 
Note that the Handler class has to be a subclass of the abstract class URLStreamHandler.
Additional Specification

      
           Networking System Properties

Since: JDK1.0/**
 * Provides the classes for implementing networking applications.
 *
 * <p> The java.net package can be roughly divided in two sections:</p>
 * <ul>
 *     <li><p><i>A Low Level API</i>, which deals with the
 *               following abstractions:</p>
 *     <ul>
 *       <li><p><i>Addresses</i>, which are networking identifiers,
 *              like IP addresses.</p></li>
 *       <li><p><i>Sockets</i>, which are basic bidirectional data communication
 *              mechanisms.</p></li>
 *       <li><p><i>Interfaces</i>, which describe network interfaces. </p></li>
 *     </ul></li>
 *     <li> <p><i>A High Level API</i>, which deals with the following
 *          abstractions:</p>
 *     <ul>
 *       <li><p><i>URIs</i>, which represent
 *               Universal Resource Identifiers.</p></li>
 *       <li><p><i>URLs</i>, which represent
 *               Universal Resource Locators.</p></li>
 *       <li><p><i>Connections</i>, which represents connections to the resource
 *               pointed to by <i>URLs</i>.</p></li>
 *       </ul></li>
 * </ul>
 * <h2>Addresses</h2>
 * <p>Addresses are used throughout the java.net APIs as either host
 *    identifiers, or socket endpoint identifiers.</p>
 * <p>The {@link java.net.InetAddress} class is the abstraction representing an
 *    IP (Internet Protocol) address.  It has two subclasses:
 * <ul>
 *       <li>{@link java.net.Inet4Address} for IPv4 addresses.</li>
 *       <li>{@link java.net.Inet6Address} for IPv6 addresses.</li>
 * </ul>
 * <p>But, in most cases, there is no need to deal directly with the subclasses,
 *    as the InetAddress abstraction should cover most of the needed
 *    functionality.</p>
 * <h3><b>About IPv6</b></h3>
 * <p>Not all systems have support for the IPv6 protocol, and while the Java
 *    networking stack will attempt to detect it and use it transparently when
 *    available, it is also possible to disable its use with a system property.
 *    In the case where IPv6 is not available, or explicitly disabled,
 *    Inet6Address are not valid arguments for most networking operations any
 *    more. While methods like {@link java.net.InetAddress#getByName} are
 *    guaranteed not to return an Inet6Address when looking up host names, it
 *    is possible, by passing literals, to create such an object. In which
 *    case, most methods, when called with an Inet6Address will throw an
 *    Exception.</p>
 * <h2>Sockets</h2>
 * <p>Sockets are means to establish a communication link between machines over
 *    the network. The java.net package provides 4 kinds of Sockets:</p>
 * <ul>
 *       <li>{@link java.net.Socket} is a TCP client API, and will typically
 *            be used to {@linkplain java.net.Socket#connect(SocketAddress)
 *            connect} to a remote host.</li>
 *       <li>{@link java.net.ServerSocket} is a TCP server API, and will
 *            typically {@linkplain java.net.ServerSocket#accept accept}
 *            connections from client sockets.</li>
 *       <li>{@link java.net.DatagramSocket} is a UDP endpoint API and is used
 *            to {@linkplain java.net.DatagramSocket#send send} and
 *            {@linkplain java.net.DatagramSocket#receive receive}
 *            {@linkplain java.net.DatagramPacket datagram packets}.</li>
 *       <li>{@link java.net.MulticastSocket} is a subclass of
 *            {@code DatagramSocket} used when dealing with multicast
 *            groups.</li>
 * </ul>
 * <p>Sending and receiving with TCP sockets is done through InputStreams and
 *    OutputStreams which can be obtained via the
 *    {@link java.net.Socket#getInputStream} and
 *    {@link java.net.Socket#getOutputStream} methods.</p>
 * <h2>Interfaces</h2>
 * <p>The {@link java.net.NetworkInterface} class provides APIs to browse and
 *    query all the networking interfaces (e.g. ethernet connection or PPP
 *    endpoint) of the local machine. It is through that class that you can
 *    check if any of the local interfaces is configured to support IPv6.</p>
 * <p>Note, all conforming implementations must support at least one
 *    {@code NetworkInterface} object, which must either be connected to a
 *    network, or be a "loopback" interface that can only communicate with
 *    entities on the same machine.</p>
 *
 * <h2>High level API</h2>
 * <p>A number of classes in the java.net package do provide for a much higher
 *    level of abstraction and allow for easy access to resources on the
 *    network. The classes are:
 * <ul>
 *       <li>{@link java.net.URI} is the class representing a
 *            Universal Resource Identifier, as specified in RFC 2396.
 *            As the name indicates, this is just an Identifier and doesn't
 *            provide directly the means to access the resource.</li>
 *       <li>{@link java.net.URL} is the class representing a
 *            Universal Resource Locator, which is both an older concept for
 *            URIs and a means to access the resources.</li>
 *       <li>{@link java.net.URLConnection} is created from a URL and is the
 *            communication link used to access the resource pointed by the
 *            URL. This abstract class will delegate most of the work to the
 *            underlying protocol handlers like http or https.</li>
 *       <li>{@link java.net.HttpURLConnection} is a subclass of URLConnection
 *            and provides some additional functionalities specific to the
 *            HTTP protocol.</li>
 * </ul>
 * <p>The recommended usage is to use {@link java.net.URI} to identify
 *    resources, then convert it into a {@link java.net.URL} when it is time to
 *    access the resource. From that URL, you can either get the
 *    {@link java.net.URLConnection} for fine control, or get directly the
 *    InputStream.
 * <p>Here is an example:</p>
 * <pre>
 * URI uri = new URI("http://java.sun.com/");
 * URL url = uri.toURL();
 * InputStream in = url.openStream();
 * </pre>
 * <h2>Protocol Handlers</h2>
 * As mentioned, URL and URLConnection rely on protocol handlers which must be
 * present, otherwise an Exception is thrown. This is the major difference with
 * URIs which only identify resources, and therefore don't need to have access
 * to the protocol handler. So, while it is possible to create an URI with any
 * kind of protocol scheme (e.g. {@code myproto://myhost.mydomain/resource/}),
 * a similar URL will try to instantiate the handler for the specified protocol;
 * if it doesn't exist an exception will be thrown.
 * <p>By default the protocol handlers are loaded dynamically from the default
 *    location. It is, however, possible to add to the search path by setting
 *    the {@code java.protocol.handler.pkgs} system property. For instance if
 *    it is set to {@code myapp.protocols}, then the URL code will try, in the
 *    case of http, first to load {@code myapp.protocols.http.Handler}, then,
 *    if this fails, {@code http.Handler} from the default location.
 * <p>Note that the Handler class <b>has to</b> be a subclass of the abstract
 *    class {@link java.net.URLStreamHandler}.</p>
 * <h2>Additional Specification</h2>
 * <ul>
 *       <li><a href="doc-files/net-properties.html">
 *            Networking System Properties</a></li>
 * </ul>
 *
 * @since JDK1.0
 */
package java.net;
/

java/ 8/ java/net/package-info.java

Addresses

About IPv6

Sockets

Interfaces

High level API

Protocol Handlers

Additional Specification
