Who is connected to your windows machine using which port.

To access any application of another system either through WWW service, FTP service, POP3/SMTP services, or even just NetBIOS over TCP/IP you have to go through a port. In other sense if someone from outside access something on your system he enter through a port. And you sometimes might be curious to know who is accessing your system and which ports they are using.
With netstat tool you can easily identify it.
On windows system in order to know details of it just type netstat /? in command prompt.
Here is the output,

C:\Documents and Settings\Queen>netstat /?

Displays protocol statistics and current TCP/IP network connections.

NETSTAT [-a] [-b] [-e] [-n] [-o] [-p proto] [-r] [-s] [-v] [interval]

-a Displays all connections and listening ports.
-b Displays the executable involved in creating each connection or
listening port. In some cases well-known executables host
multiple independent components, and in these cases the
sequence of components involved in creating the connection
or listening port is displayed. In this case the executable
name is in [] at the bottom, on top is the component it called,
and so forth until TCP/IP was reached. Note that this option
can be time-consuming and will fail unless you have sufficient
permissions.
-e Displays Ethernet statistics. This may be combined with the -s
option.
-n Displays addresses and port numbers in numerical form.
-o Displays the owning process ID associated with each connection.
-p proto Shows connections for the protocol specified by proto; proto
may be any of: TCP, UDP, TCPv6, or UDPv6. If used with the -s
option to display per-protocol statistics, proto may be any of:
IP, IPv6, ICMP, ICMPv6, TCP, TCPv6, UDP, or UDPv6.
-r Displays the routing table.
-s Displays per-protocol statistics. By default, statistics are
shown for IP, IPv6, ICMP, ICMPv6, TCP, TCPv6, UDP, and UDPv6;
the -p option may be used to specify a subset of the default.
-v When used in conjunction with -b, will display sequence of
components involved in creating the connection or listening
port for all executables.
interval Redisplays selected statistics, pausing interval seconds
between each display. Press CTRL+C to stop redisplaying
statistics. If omitted, netstat will print the current
configuration information once.

In order to know all connection along with port now connection to your system just press netstat.
C:\Documents and Settings\Queen>netstat
The listing will appear with several fields.
-The protocol type.
-Hostname:port
-Foreign Address that is who is connected to your system.
-State of the connection.

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