Ubuntu - IPTables - Reset

iptables -F
iptables -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -P INPUT ACCEPT
iptables -P FORWARD ACCEPT
iptables -P OUTPUT ACCEPT

NOTE: The ACCEPT in the last 3 commands allows any traffic (inbound, outbound, forward).

To make it more secure, these could be changed to DENY.