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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 Inet4Address#format; For IPv6 address format, please refer to Inet6Address#format.

There is 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.

By default, when a security manager is installed, in order to protect against DNS spoofing attacks, the result of positive host name resolutions are cached forever. When a security manager is not installed, 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

See Also:

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 configured system 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.
`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.
`String`	`toString()`	Converts this IP address to a `String`.

Methods declared in class java.lang.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.

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 configured resolver. If a lookup of the name service is required, call getCanonicalHostName.

If there is a security manager, its checkConnect method is first called with the hostname and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, it will return the textual representation of the IP address.

Returns:

the host name for this IP address, or if the operation is not allowed by the security check, the textual representation of the IP address.

See Also:

getCanonicalHostName

public String 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.

If there is a security manager, this method first calls its checkConnect method with the hostname and -1 as its arguments to see if the calling code is allowed to know the hostname for this IP address, i.e., to connect to the host. If the operation is not allowed, it will return the textual representation of the IP address.

Returns:

the fully qualified domain name for this IP address, or if the operation is not allowed by the security check, the textual representation of the IP address.

Since:

1.4

See Also:

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:

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:

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.

If there is a security manager, and host is not null or host.length() is not equal to zero, the security manager's checkConnect method is called with the hostname and -1 as its arguments to determine if the operation is allowed.

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.

SecurityException - if a security manager exists and its checkConnect method doesn't allow the operation

External Specifications

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 configured system resolver.

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.

SecurityException - if a security manager exists and its checkConnect method doesn't allow the operation.

External Specifications

See Also:

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

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.

If there is a security manager, its checkConnect method is called with the local host name and -1 as its arguments to see if the operation is allowed. If the operation is not allowed, an InetAddress representing the loopback address is returned.

Returns:

the address of the local host.

Throws:

UnknownHostException - if the local host name could not be resolved into an address.

See Also:

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