Iptables rule

To delete existing rules:

# iptables -F

To list existing rules:

# iptables -L

To whitelist an IP address:

# iptables -A INPUT -s  <IP>  -j ACCEPT
# iptables -A OUTPUT -s  <IP> -j ACCEPT
# service iptables save

To blacklist an IP address:

# iptables -I INPUT -j DROP -s <IP>
# iptables -I OUTPUT -j DROP -s <IP>
# service iptables save

To unblock an IP address:

# iptables -D INPUT -s <IP> -j DROP
# iptables -D INPUT -s <IP> -j DROP
# service iptables save

To open a port range(like 3000 to 5000):

# iptables -A INPUT -p tcp --destination-port 3000:5000 -j ACCEPT
# iptables -A OUTPUT -p tcp --source-port 3000:5000 -j ACCEPT
# service iptables save

To redirect all traffic from one IP address to another IP address:

# iptables -t nat -A PREROUTING -d <FIRST IP> -j DNAT --to-destination <ANOTHER IP>
# service iptables save

To allow TCP port 25 for all:

# iptables -A INPUT -m state --state NEW -p tcp --dport 25 -j ACCEPT

 Note:
-A -> Append,
-I -> Insert,
-D -> Delete,
-j -> Jump to target,
-i eth0 -> interface eth0,
-p tcp -> protocol TCP,
–dport -> destination port

Thank you.