The internal side of my ISP router has three devices: ISP router 128.128.43.1 Firewall router 128.128.43.2 Server 128.128.43.3 Behind the Firewall router is a NAT network using 192.168.100.n/24 This question is regarding iptables running on the Server. I wanted to allow access to port 8080 only from the NAT clients behind the Firewall router, so I used this rule -A Firewall-1-INPUT -s 192.168.100.0/24 -m state --state NEW -m tcp -p tcp --dport 8080 -j ACCEPT This worked, but UNEXPECTEDLY ALLOWED GLOBAL ACCESS, which resulted in our JBOSS server getting compromised. I now know that the correct rule is to use the Firewall router's address instead of the internal network, but can anyone explain why the first rule allowed global access? I would have expected it to just fail. Full config, mostly lifted from a RedHat server: *filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] :Firewall-1-INPUT - [0:0] -A INPUT -j Firewall-1-INPUT -A FORWARD -j Firewall-1-INPUT -A Firewall-1-INPUT -i lo -j ACCEPT -A Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT -A Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A Firewall-1-INPUT -m comment --comment "allow ssh from all" -A Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A Firewall-1-INPUT -m comment --comment "allow https from all" -A Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 443 -j ACCEPT -A Firewall-1-INPUT -m comment --comment "allow JBOSS from Firewall" ### THIS RESULTED IN GLOBAL ACCESS TO PORT 8080 ### -A Firewall-1-INPUT -s 192.168.100.0/24 -m state --state NEW -m tcp -p tcp --dport 8080 -j ACCEPT ### THIS WORKED -A Firewall-1-INPUT -s 128.128.43.2 -m state --state NEW -m tcp -p tcp --dport 8080 -j ACCEPt ### -A Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited COMMIT