# Class InetAddress

java.lang.Object
java.net.InetAddress

All Implemented Interfaces:
  `Serializable`

Direct Known Subclasses:
  `Inet4Address, Inet6Address`

public sealed class InetAddress extends Object implements Serializable permits Inet4Address, Inet6Address
This class represents an Internet Protocol (IP) address.

An IP address is either a 32-bit or 128-bit unsigned number used by IP, a lower-level protocol on which protocols like UDP and TCP are built. The IP address architecture is defined by RFC 790: Assigned Numbers, RFC 1918: Address Allocation for Private Internets, RFC 2365: Administratively Scoped IP Multicast, and RFC 2373: IP Version 6 Addressing Architecture. An instance of an InetAddress consists of an IP address and possibly its corresponding host name (depending on whether it is constructed with a host name or whether it has already done reverse host name resolution).

## Address types

Description of unicast and multicast address types
Address Type | Description
--- | ---
unicast | An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. The Unspecified Address -- Also called anylocal or wildcard address. It must never be assigned to any node. It indicates the absence of an address. One example of its use is as the target of bind, which allows a server to accept a client connection on any interface, in case the server host has multiple interfaces. The unspecified address must not be used as the destination address of an IP packet. The Loopback Addresses -- This is the address assigned to the loopback interface. Anything sent to this IP address loops around and becomes IP input on the local host. This address is often used when testing a client.
multicast | An identifier for a set of interfaces (typically belonging to different nodes). A packet sent to a multicast address is delivered to all interfaces identified by that address.

### IP address scope

Link-local addresses are designed to be used for addressing on a single link for purposes such as auto-address configuration, neighbor discovery, or when no routers are present.

Site-local addresses are designed to be used for addressing inside of a site without the need for a global prefix.

Global addresses are unique across the internet.

### Textual representation of IP addresses

The textual representation of an IP address is address family specific.

For IPv4 address format, please refer to the supported IPv4 address textual representations; For IPv6 address format, please refer to the supported IPv6 address textual representations.

There are a couple of System Properties affecting how IPv4 and IPv6 addresses are used.

## Host Name Resolution

The InetAddress class provides methods to resolve host names to their IP addresses and vice versa. The actual resolution is delegated to an InetAddress resolver.

Host name-to-IP address resolution maps a host name to an IP address. For any host name, its corresponding IP address is returned.

Reverse name resolution means that for any IP address, the host associated with the IP address is returned.

The built-in InetAddress resolver implementation does host name-to-IP address resolution and vice versa through the use of a combination of local machine configuration information and network naming services such as the Domain Name System (DNS) and the Lightweight Directory Access Protocol (LDAP). The particular naming services that the built-in resolver uses by default depends on the configuration of the local machine.

`InetAddress` has a service provider mechanism for InetAddress resolvers that allows a custom InetAddress resolver to be used instead of the built-in implementation. `InetAddressResolverProvider` is the service provider class. Its API docs provide all the details on this mechanism.

## InetAddress Caching

The InetAddress class has a cache to store successful as well as unsuccessful host name resolutions.

The default behavior is to cache entries for a finite (implementation dependent) period of time. The result of unsuccessful host name resolution is cached for a very short period of time (10 seconds) to improve performance.

If the default behavior is not desired, then a Java security property can be set to a different Time-to-live (TTL) value for positive caching. Likewise, a system admin can configure a different negative caching TTL value when needed or extend the usage of the stale data.

Three Java security properties control the TTL values used for positive and negative host name resolution caching:

networkaddress.cache.ttl
  Indicates the caching policy for successful name lookups from the name service. The value is specified as an integer to indicate the number of seconds to cache the successful lookup. The default setting is to cache for an implementation specific period of time.

  A value of -1 indicates "cache forever".

networkaddress.cache.stale.ttl
  Indicates the caching policy for stale names. The value is specified as an integer to indicate the number of seconds that stale names will be kept in the cache. A name is considered stale if the TTL has expired and an attempt to lookup the host name again was not successful. This property is useful if it is preferable to use a stale name rather than fail due to an unsuccessful lookup. The default setting is to cache for an implementation specific period of time.

  If the value of this property is larger than "networkaddress.cache.ttl" then "networkaddress.cache.ttl" will be used as a refresh interval of the name in the cache. For example, if this property is set to 1 day and "networkaddress.cache.ttl" is set to 30 seconds, then the positive response will be cached for 1 day but an attempt to refresh it will be done every 30 seconds.

  A value of 0 (zero) or if the property is not set means do not use stale names. Negative values are ignored.

networkaddress.cache.negative.ttl (default: 10)
  Indicates the caching policy for un-successful name lookups from the name service. The value is specified as an integer to indicate the number of seconds to cache the failure for un-successful lookups.

  A value of 0 indicates "never cache". A value of -1 indicates "cache forever".

Sealed Class Hierarchy Graph:
Since:
  1.0
External Specifications
  - RFC 1918: Address Allocation for Private Internets
  - RFC 2365: Administratively Scoped IP Multicast
  - RFC 2373: IP Version 6 Addressing Architecture
  - RFC 790: Assigned numbers
See Also:
  - `getByAddress(byte[])`
  - `getByAddress(java.lang.String, byte[])`
  - `getAllByName(java.lang.String)`
  - `getByName(java.lang.String)`
  - `getLocalHost()`
  - Serialized Form

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `equals(Object obj)`
  Compares this object against the specified object.
  `byte[]`
  `getAddress()`
  Returns the raw IP address of this `InetAddress` object.
  `static InetAddress[]`
  `getAllByName(String host)`
  Given the name of a host, returns an array of its IP addresses, based on the system-wide resolver.
  `static InetAddress`
  `getByAddress(byte[] addr)`
  Returns an `InetAddress` object given the raw IP address .
  `static InetAddress`
  `getByAddress(String host, byte[] addr)`
  Creates an InetAddress based on the provided host name and IP address.
  `static InetAddress`
  `getByName(String host)`
  Determines the IP address of a host, given the host's name.
  `String`
  `getCanonicalHostName()`
  Gets the fully qualified domain name for this IP address using the system-wide resolver.
  `String`
  `getHostAddress()`
  Returns the IP address string in textual presentation.
  `String`
  `getHostName()`
  Gets the host name for this IP address.
  `static InetAddress`
  `getLocalHost()`
  Returns the address of the local host.
  `static InetAddress`
  `getLoopbackAddress()`
  Returns the loopback address.
  `int`
  `hashCode()`
  Returns a hashcode for this IP address.
  `boolean`
  `isAnyLocalAddress()`
  Utility routine to check if the InetAddress is a wildcard address.
  `boolean`
  `isLinkLocalAddress()`
  Utility routine to check if the InetAddress is a link local address.
  `boolean`
  `isLoopbackAddress()`
  Utility routine to check if the InetAddress is a loopback address.
  `boolean`
  `isMCGlobal()`
  Utility routine to check if the multicast address has global scope.
  `boolean`
  `isMCLinkLocal()`
  Utility routine to check if the multicast address has link scope.
  `boolean`
  `isMCNodeLocal()`
  Utility routine to check if the multicast address has node scope.
  `boolean`
  `isMCOrgLocal()`
  Utility routine to check if the multicast address has organization scope.
  `boolean`
  `isMCSiteLocal()`
  Utility routine to check if the multicast address has site scope.
  `boolean`
  `isMulticastAddress()`
  Utility routine to check if the InetAddress is an IP multicast address.
  `boolean`
  `isReachable(int timeout)`
  Test whether that address is reachable.
  `boolean`
  `isReachable(NetworkInterface netif, int ttl, int timeout)`
  Test whether that address is reachable.
  `boolean`
  `isSiteLocalAddress()`
  Utility routine to check if the InetAddress is a site local address.
  `static InetAddress`
  `ofLiteral(String ipAddressLiteral)`
  Creates an `InetAddress` based on the provided textual representation of an IP address.
  `String`
  `toString()`
  Converts this IP address to a `String`.

  ### Methods declared in class Object

  `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- ## Method Details

  - ### isMulticastAddress

    public boolean isMulticastAddress()
    Utility routine to check if the InetAddress is an IP multicast address.
    Returns:
      a `boolean` indicating if the InetAddress is an IP multicast address
    Since:
      1.1

  - ### isAnyLocalAddress

    public boolean isAnyLocalAddress()
    Utility routine to check if the InetAddress is a wildcard address.
    Returns:
      a `boolean` indicating if the InetAddress is a wildcard address.
    Since:
      1.4

  - ### isLoopbackAddress

    public boolean isLoopbackAddress()
    Utility routine to check if the InetAddress is a loopback address.
    Returns:
      a `boolean` indicating if the InetAddress is a loopback address; or false otherwise.
    Since:
      1.4

  - ### isLinkLocalAddress

    public boolean isLinkLocalAddress()
    Utility routine to check if the InetAddress is a link local address.
    Returns:
      a `boolean` indicating if the InetAddress is a link local address; or false if address is not a link local unicast address.
    Since:
      1.4

  - ### isSiteLocalAddress

    public boolean isSiteLocalAddress()
    Utility routine to check if the InetAddress is a site local address.
    Returns:
      a `boolean` indicating if the InetAddress is a site local address; or false if address is not a site local unicast address.
    Since:
      1.4

  - ### isMCGlobal

    public boolean isMCGlobal()
    Utility routine to check if the multicast address has global scope.
    Returns:
      a `boolean` indicating if the address has is a multicast address of global scope, false if it is not of global scope or it is not a multicast address
    Since:
      1.4

  - ### isMCNodeLocal

    public boolean isMCNodeLocal()
    Utility routine to check if the multicast address has node scope.
    Returns:
      a `boolean` indicating if the address has is a multicast address of node-local scope, false if it is not of node-local scope or it is not a multicast address
    Since:
      1.4

  - ### isMCLinkLocal

    public boolean isMCLinkLocal()
    Utility routine to check if the multicast address has link scope.
    Returns:
      a `boolean` indicating if the address has is a multicast address of link-local scope, false if it is not of link-local scope or it is not a multicast address
    Since:
      1.4

  - ### isMCSiteLocal

    public boolean isMCSiteLocal()
    Utility routine to check if the multicast address has site scope.
    Returns:
      a `boolean` indicating if the address has is a multicast address of site-local scope, false if it is not of site-local scope or it is not a multicast address
    Since:
      1.4

  - ### isMCOrgLocal

    public boolean isMCOrgLocal()
    Utility routine to check if the multicast address has organization scope.
    Returns:
      a `boolean` indicating if the address has is a multicast address of organization-local scope, false if it is not of organization-local scope or it is not a multicast address
    Since:
      1.4

  - ### isReachable

    public boolean isReachable(int timeout) throws IOException
    Test whether that address is reachable. Best effort is made by the implementation to try to reach the host, but firewalls and server configuration may block requests resulting in an unreachable status while some specific ports may be accessible. A typical implementation will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host.

    The timeout value, in milliseconds, indicates the maximum amount of time the try should take. If the operation times out before getting an answer, the host is deemed unreachable. A negative value will result in an IllegalArgumentException being thrown.

    Parameters:
      `timeout` - the time, in milliseconds, before the call aborts
    Returns:
      a `boolean` indicating if the address is reachable.
    Throws:
      `IOException` - if a network error occurs
      `IllegalArgumentException` - if `timeout` is negative.
    Since:
      1.5

  - ### isReachable

    public boolean isReachable(NetworkInterface netif, int ttl, int timeout) throws IOException
    Test whether that address is reachable. Best effort is made by the implementation to try to reach the host, but firewalls and server configuration may block requests resulting in a unreachable status while some specific ports may be accessible. A typical implementation will use ICMP ECHO REQUESTs if the privilege can be obtained, otherwise it will try to establish a TCP connection on port 7 (Echo) of the destination host.

    The `network interface` and `ttl` parameters let the caller specify which network interface the test will go through and the maximum number of hops the packets should go through. A negative value for the `ttl` will result in an IllegalArgumentException being thrown.

    The timeout value, in milliseconds, indicates the maximum amount of time the try should take. If the operation times out before getting an answer, the host is deemed unreachable. A negative value will result in an IllegalArgumentException being thrown.

    Parameters:
      `netif` - the NetworkInterface through which the test will be done, or null for any interface
      `ttl` - the maximum numbers of hops to try or 0 for the default
      `timeout` - the time, in milliseconds, before the call aborts
    Returns:
      a `boolean` indicating if the address is reachable.
    Throws:
      `IllegalArgumentException` - if either `timeout` or `ttl` are negative.
      `IOException` - if a network error occurs
    Since:
      1.5

  - ### getHostName

    public String getHostName()
    Gets the host name for this IP address.

    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-wide resolver. If a lookup of the name service is required, call `getCanonicalHostName`.

    Returns:
      the host name for this IP address
    See Also:
      - `getCanonicalHostName()`

  - ### getCanonicalHostName

    public String getCanonicalHostName()
    Gets the fully qualified domain name for this IP address using the system-wide resolver.

    The system-wide resolver will be used to do a reverse name lookup of the IP address. The lookup can fail for many reasons that include the host not being registered with the name service. If the resolver is unable to determine the fully qualified domain name, this method returns the textual representation of the IP address.

    Returns:
      the fully qualified domain name for this IP address. If the system-wide resolver wasn't able to determine the fully qualified domain name for the IP address, the textual representation of the IP address is returned instead.
    Since:
      1.4

  - ### getAddress

    public byte[] getAddress()
    Returns the raw IP address of this `InetAddress` object. The result is in network byte order: the highest order byte of the address is in `getAddress()[0]`.
    Returns:
      the raw IP address of this object.

  - ### getHostAddress

    public String getHostAddress()
    Returns the IP address string in textual presentation.
    Returns:
      the raw IP address in a string format.
    Since:
      1.0.2

  - ### hashCode

    public int hashCode()
    Returns a hashcode for this IP address.
    Overrides:
      `hashCode` in class `Object`
    Returns:
      a hash code value for this IP address.
    See Also:
      - `Object.equals(java.lang.Object)`
      - `System.identityHashCode(Object)`

  - ### equals

    public boolean equals(Object obj)
    Compares this object against the specified object. The result is `true` if and only if the argument is not `null` and it represents the same IP address as this object.

    Two instances of `InetAddress` represent the same IP address if the length of the byte arrays returned by `getAddress` is the same for both, and each of the array components is the same for the byte arrays.

    Overrides:
      `equals` in class `Object`
    Parameters:
      `obj` - the object to compare against.
    Returns:
      `true` if the objects are the same; `false` otherwise.
    See Also:
      - `getAddress()`

  - ### toString

    public String toString()
    Converts this IP address to a `String`. The string returned is of the form: hostname / literal IP address. If the host name is unresolved, no reverse lookup is performed. The hostname part will be represented by an empty string.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of this IP address.

  - ### getByAddress

    public static InetAddress getByAddress(String host, byte[] addr) throws UnknownHostException
    Creates an InetAddress based on the provided host name and IP address. The system-wide resolver is not used to check the validity of the address.

    The host name can either be a machine name, such as "`www.example.com`", or a textual representation of its IP address.

    No validity checking is done on the host name either.

    If addr specifies an IPv4 address an instance of Inet4Address will be returned; otherwise, an instance of Inet6Address will be returned.

    IPv4 address byte array must be 4 bytes long and IPv6 byte array must be 16 bytes long

    Parameters:
      `host` - the specified host
      `addr` - the raw IP address in network byte order
    Returns:
      an InetAddress object created from the raw IP address.
    Throws:
      `UnknownHostException` - if IP address is of illegal length
    Since:
      1.4

  - ### getByName

    public static InetAddress getByName(String host) throws UnknownHostException
    Determines the IP address of a host, given the host's name.

    The host name can either be a machine name, such as "`www.example.com`", or a textual representation of its IP address. If a literal IP address is supplied, only the validity of the address format is checked.

    For `host` specified in literal IPv6 address, either the form defined in RFC 2732 or the literal IPv6 address format defined in RFC 2373 is accepted. IPv6 scoped addresses are also supported. See here for a description of IPv6 scoped addresses.

    If the host is `null` or `host.length()` is equal to zero, then an `InetAddress` representing an address of the loopback interface is returned. See RFC 3330 section 2 and RFC 2373 section 2.5.3.

    Parameters:
      `host` - the specified host, or `null`.
    Returns:
      an IP address for the given host name.
    Throws:
      `UnknownHostException` - if no IP address for the `host` could be found, or if a scope_id was specified for a global IPv6 address.
    External Specifications
      - RFC 2373: IP Version 6 Addressing Architecture
      - RFC 3330: Special-Use IPv4 Addresses

  - ### getAllByName

    public static InetAddress[] getAllByName(String host) throws UnknownHostException
    Given the name of a host, returns an array of its IP addresses, based on the system-wide resolver.

    The host name can either be a machine name, such as "`www.example.com`", or a textual representation of its IP address. If a literal IP address is supplied, only the validity of the address format is checked.

    For `host` specified in literal IPv6 address, either the form defined in RFC 2732 or the literal IPv6 address format defined in RFC 2373 is accepted. A literal IPv6 address may also be qualified by appending a scoped zone identifier or scope_id. The syntax and usage of scope_ids is described here.

    If the host is `null` or `host.length()` is equal to zero, then an `InetAddress` representing an address of the loopback interface is returned. See RFC 3330 section 2 and RFC 2373 section 2.5.3.

    Parameters:
      `host` - the name of the host, or `null`.
    Returns:
      an array of all the IP addresses for a given host name.
    Throws:
      `UnknownHostException` - if no IP address for the `host` could be found, or if a scope_id was specified for a global IPv6 address.
    External Specifications
      - RFC 2373: IP Version 6 Addressing Architecture
      - RFC 3330: Special-Use IPv4 Addresses

  - ### getLoopbackAddress

    public static InetAddress getLoopbackAddress()
    Returns the loopback address.

    The InetAddress returned will represent the IPv4 loopback address, 127.0.0.1, or the IPv6 loopback address, ::1. The IPv4 loopback address returned is only one of many in the form 127.*.*.*

    Returns:
      the InetAddress loopback instance.
    Since:
      1.7

  - ### ofLiteral

    public static InetAddress ofLiteral(String ipAddressLiteral)
    Creates an `InetAddress` based on the provided textual representation of an IP address.

    The provided IP address literal is parsed as an IPv4 address literal first. If it cannot be parsed as an IPv4 address literal, then the method attempts to parse it as an IPv6 address literal. If neither attempts succeed an `IllegalArgumentException` is thrown.

    This method doesn't block, i.e. no reverse lookup is performed.

    Parameters:
      `ipAddressLiteral` - the textual representation of an IP address.
    Returns:
      an `InetAddress` object with no hostname set, and constructed from the provided IP address literal.
    Throws:
      `IllegalArgumentException` - if the `ipAddressLiteral` cannot be parsed as an IPv4 or IPv6 address literal.
      `NullPointerException` - if the `ipAddressLiteral` is `null`.
    Since:
      22
    See Also:
      - `Inet4Address.ofLiteral(String)`
      - `Inet6Address.ofLiteral(String)`
      - `Inet4Address.ofPosixLiteral(String)`

  - ### getByAddress

    public static InetAddress getByAddress(byte[] addr) throws UnknownHostException
    Returns an `InetAddress` object given the raw IP address . The argument is in network byte order: the highest order byte of the address is in `getAddress()[0]`.

    This method doesn't block, i.e. no reverse lookup is performed.

    IPv4 address byte array must be 4 bytes long and IPv6 byte array must be 16 bytes long

    Parameters:
      `addr` - the raw IP address in network byte order
    Returns:
      an InetAddress object created from the raw IP address.
    Throws:
      `UnknownHostException` - if IP address is of illegal length
    Since:
      1.4

  - ### getLocalHost

    public static InetAddress getLocalHost() throws UnknownHostException
    Returns the address of the local host. This is achieved by retrieving the name of the host from the system, then resolving that name into an `InetAddress`.

    Note: The resolved address may be cached for a short period of time.

    Returns:
      the address of the local host.
    Throws:
      `UnknownHostException` - if the local host name could not be resolved into an address.
    See Also:
      - `getByName(java.lang.String)`