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
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.
Host Name Resolution
Host name-to-IP address
resolution is accomplished through
the use of a combination of local machine configuration information
and network naming services such as the Domain Name System (DNS)
and Network Information Service(NIS). The particular naming
services(s) being used is by default the local machine configured
one. 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 InetAddress class provides methods to resolve host names to
their IP addresses and vice versa.
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.
Two 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 as 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.negative.ttl (default: 10)
- Indicates the caching policy for un-successful name lookups
from the name service. The value is specified as as 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".