Why does the internet disconnect after clearing the iptables rules?

Hello,
My current iptables rules are:
Code:$ sudo iptables -S
-P INPUT DROP
-P FORWARD DROP
-P OUTPUT ACCEPT
-N SYN_FLOOD
-A INPUT -s 127.0.0.0/8 -d 127.0.0.0/8 -i lo -j ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -j REJECT --reject-with icmp-port-unreachable
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j SYN_FLOOD
-A INPUT -p tcp -m tcp --dport 9050 -j ACCEPT
-A FORWARD -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -s 192.168.56.0/24 -i vboxnet0 -o eth1 -m conntrack --ctstate NEW -j ACCEPT
-A FORWARD -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
-A OUTPUT -p tcp -j ACCEPT
-A OUTPUT -p udp -j ACCEPT
-A OUTPUT -p icmp -j ACCEPT
-A OUTPUT -j REJECT --reject-with icmp-port-unreachable
-A OUTPUT -o lo -j ACCEPT
-A SYN_FLOOD -m limit --limit 5/sec --limit-burst 10 -j RETURN
-A SYN_FLOOD -j DROPWhen I flushed these rules, then the Internet will be disconnected. Why?
In Windows OS, when you disable the firewall, then the Internet is not disconnected.

Thank you.