Interacting with Networks and Databases - Beginning Java 7 - page 789

dress(see http://en.wikipedia.org/wiki/IPv4 ) .Similarly,a128-bitIP
addressisoftenreferredtoasanInternetProtocolVersion6(IPv6)address(see ht-
tp://en.wikipedia.org/wiki/IPv6 ) .
Port numbers are 16-bit unsigned integers that uniquely identify processes, which
are the sources orrecipients ofmessages. Portnumbers less than 1024are reserved
for standard processes. For example, port number 25 has traditionally identified
the Simple Mail Transfer Protocol (SMTP) process for sending email, although
port number 587 is commonly being used these days (see ht-
tp://en.wikipedia.org/wiki/Smtp ) .
TCP is used to create an ongoing conversation between two hosts by sending mes-
sages back and forth. Before this conversation can occur, a connection must be estab-
lishedbetweenthesehosts.Afterthisconnectionhasbeenestablished,TCPentersapat-
ternofsendingamessagepacketandwaitingforareplythatthepacketarrivedcorrectly
(orforatimeouttoexpirewhenthereplydoesn'tarrivebecauseofanetworkproblem).
This send/reply cycle guarantees a reliable connection.
Because it can take time to establish a connection, and because it also takes time to
sendpacketsbecauseoftheneedtoreceivereplyacknowledgments(ortimeouts),TCP
isfairlyslow.UDP,whichdoesn'trequireconnectionsandpacketacknowledgement,is
muchfasterthanTCP.However,UDPisn'tasreliable(there'snoguaranteethatapacket
will arrive correctly or even arrive) as TCP because there's no acknowledgment. Fur-
thermore, UDP is limited to single-packet one-way conversations.
The java.net package provides Socket and ServerSocket classes for per-
forming TCP-based communications. It also provides DatagramSocket , Data-
gramPacket , and MulticastSocket classes for performing UDP communica-
tions. MulticastSocket is a subclass of DatagramSocket .
Socket Addresses
Instancesofthe Socket -suffixedclassesareassociatedwith socket addresses thatare
comprised of IP addresses and port numbers.
The Socket classreliesonthe java.net.InetAddress classtorepresentthe
IPv4orIPv6addressportionofthesocketaddress.Itrepresentstheportnumbersepar-
ately. (The other Socket -suffixed classes also take advantage of InetAddress .)
Note InetAddress reliesonits java.net.Inet4Address subclasstorep-
resentanIPv4addressandonits java.net.Inet6Address subclasstorepresent
an IPv6 address.