查看 某端口是否可以访问 ：
telnet 100.10.20.21 8081
查看开放的端口：
more /etc/sysconfig/iptables
查看效果 ：
[root@localhost bin]# more /etc/sysconfig/iptables
Firewall configuration written by system-config-firewall
Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -m state –state NEW -m tcp -p tcp –dport 22 -j ACCEPT
-A INPUT -m state –state NEW -m tcp -p tcp –dport 18081 -j ACCEPT
-A INPUT -m state –state NEW -m tcp -p tcp –dport 8080 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 80 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 443 -j ACCEPT
-A INPUT -m state –state NEW -m udp -p udp –dport 161 -j ACCEPT
-A INPUT -j REJECT –reject-with icmp-host-prohibited
-A FORWARD -j REJECT –reject-with icmp-host-prohibited
COMMIT
[root@localhost bin]#
开放某端口：
iptables -I INPUT -p tcp –dport 8889 -j ACCEPT
禁止端口：
iptables -I INPUT -p tcp –dport 8889 -j DROP