Module java.base

Class InetAddress

All Implemented Interfaces:
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 TypeDescription
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:

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".

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:
Sealed class hierarchy graph for InetAddressSealed class hierarchy graph for InetAddress
External Specifications
See Also: