Sockets for Servers - Java Network Programming - page 303

throws IOException
public ServerSocket () throws IOException
These constructors specify the port, the length of the queue used to hold incoming
connection requests, and the local network interface to bind to. They pretty much all
do the same thing, though some use default values for the queue length and the address
to bind to.
For example, to create a server socket that would be used by an HTTP server on port
80, you would write:
ServerSocket httpd = new ServerSocket ( 80 );
To create a server socket that would be used by an HTTP server on port 80 and queues
up to 50 unaccepted connections at a time:
ServerSocket httpd = new ServerSocket ( 80 , 50 );
If you try to expand the queue past the operating system's maximum queue length, the
maximum queue length is used instead.
By default, if a host has multiple network interfaces or IP addresses, the server socket
listens on the specified port on all the interfaces and IP addresses. However, you can
add a third argument to bind only to one particular local IP address. That is, the server
socket only listens for incoming connections on the specified address; it won't listen for
connections that come in through the host's other addresses.
For example, login.ibiblio.org is a particular Linux box in North Carolina. It's connected
to the Internet with the IP address 152.2.210.122. The same box has a second Ethernet
card with the local IP address 192.168.210.122 that is not visible from the public Internet,
only from the local network. If, for some reason, you wanted to run a server on this host
that only responded to local connections from within the same network, you could
create a server socket that listens on port 5776 of 192.168.210.122 but not on port 5776
of 152.2.210.122, like so:
InetAddress local = InetAddress . getByName ( "192.168.210.122" );
ServerSocket httpd = new ServerSocket ( 5776 , 10 , local );
In all three constructors, you can pass 0 for the port number so the system will select
an available port for you. A port chosen by the system like this is sometimes called an
anonymous port because you don't know its number in advance (though you can find
out after the port has been chosen). This is often useful in multisocket protocols such
as FTP. In passive FTP the client first connects to a server on the well-known port 21,
so the server has to specify that port. However, when a file needs to be transferred, the
server starts listening on any available port. The server then tells the client what other
port it should connect to for data using the command connection already open on port
21. Thus, the data port can change from one session to the next and does not need to
be known in advance. (Active FTP is similar except the client listens on an ephemeral
port for the server to connect to it, rather than the other way around.)