commandline session
$nc 127.0.0.1 25 220 debian.Jeff ESMTP Exim 4.80 Thu, 27 Jun 2013 00:05:20 +0530 ^C $nc gnu.org no port[s] to connect to $nc gnu.org 25 ^C $nc -g gnu.org 25 no port[s] to connect to $nc -g gnu.org gnu.org 25 ^C $nc -g 127.0.0.1 127.0.0.1 25 ^C $nc www.youtube.com no port[s] to connect to $nc www.youtube.com 80 ^C $ $nc -z -vv www.gnu.org 80 wildebeest.gnu.org [208.118.235.148] 80 (http) open sent 0, rcvd 0 $nc -z -vv 127.0.0.1 80 localhost.localdomain [127.0.0.1] 80 (http) : Connection refused sent 0, rcvd 0 $nc -z -vv 127.0.0.1 25 localhost.localdomain [127.0.0.1] 25 (smtp) open sent 0, rcvd 0 $