http://unix.stackexchange.com/questions/1911/what-is-the-loopback-interface

The loopback networking interface is a virtual network device
implemented entirely in software. All traffic sent to it "loops back"
and just targets services on your local machine.

eth0 tends to be the name of the first hardware network device (on
linux, at least), and will send network traffic to remote
machines. You might see it as en0, ent0, et0, or various other names
depending on which OS you're using at the time. (It could also be a
virtual device, but that's another topic)

The loopback option used when mounting an ISO image has nothing to do
with the networking interface, it just means that the mount command
has to first associate the file with a device node (/dev/loopback or
something with a similar name) before mounting it to the target
directory. It "loops back" reads (and writes, if supported) to a file
on an existing mount, instead of using a device directly.

State The state of the socket. Since there are no states in raw mode and usually no
 states used in UDP, this column may be left blank. Normally this can be one of
 several values:

 ESTABLISHED The socket has an established connection.

 SYN_SENT The socket is actively attempting to establish a connection.

 SYN_RECV A connection request has been received from the network.

 FIN_WAIT1 The socket is closed, and the connection is shutting down.

 FIN_WAIT2 Connection is closed, and the socket is waiting for a
        shutdown from the remote end.

 TIME_WAIT The socket is waiting after close to handle packets still
 in the network.

 CLOSE The socket is not being used.

 CLOSE_WAIT The remote end has shut down, waiting for the socket to
 close.

 LAST_ACK The remote end has shut down, and the socket is
        closed. Waiting for acknowledgement.

 LISTEN The socket is listening for incoming connections.  Such
        sockets are not included in the output unless you specify the
        --listening (-l) or --all (-a) option.

 CLOSING Both sockets are shut down but we still don't have all our
 data sent.

 UNKNOWN The state of the socket is unknown.

http://www.cnblogs.com/zhenjing/archive/2011/04/20/2021791.html

[root@localhost i386]# netstat -nlp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address        Foreign Address      State       PID/Program name
tcp  0   0 0.0.0.0:3306          0.0.0.0:*       LISTEN      2804/mysqld
tcp  0   0 222.230.14.16:80    0.0.0.0:*       LISTEN      4433/(squid)
tcp  0   0 127.0.0.1:80          0.0.0.0:*       LISTEN      4350/httpd
tcp  0   0 127.0.0.1:2006        0.0.0.0:*       LISTEN      27724/php
tcp  0   0 0.0.0.0:25              0.0.0.0:*       LISTEN      26234/master
tcp  0   0 0.0.0.0:443            0.0.0.0:*       LISTEN      27724/php
tcp  0   0 :::22                     :::*            LISTEN      2674/sshd
udp  0   0 0.0.0.0:16384        0.0.0.0:*                      4433/(squid)
udp  0   0 0.0.0.0:3130         0.0.0.0:*                       4433/(squid)

我说的本地地址的四种类型：
0 0.0.0.0
222.230.14.16
127.0.0.1
:::
都是本地地址为何要有这四种类型呢？都是代表什么含义和作用呢？

一般 0.0.0.0 表示 all or any address , 以 listen 0.0.0.0 來說 , 表示
listen 主机所有 interface 的 ip 位址 .

至於你說的 222.230.14.16 or 127.0.0.1 那都只是表示listen特定ip位址 而已

::: 那表示 listen 主机所有 interface 的 ipv6 位址 .

http://lists.process-one.net/pipermail/tsung-users/2010-October/001552.html

http://www.cnblogs.com/zhenjing/archive/2011/04/20/2021791.html