Network Defender

Write firewall rules to let the right traffic through and keep everything else out, from ports and CIDR ranges to network segmentation.

You’re handed a small network (a web server, an API tier, a database, a VPN for remote staff) sitting behind one firewall. Traffic keeps arriving: customers, admins, and a few attackers mixed in. Write firewall rules (ALLOW or DENY, with an optional protocol, port and source range) to let the legitimate traffic through and block the rest. Rules are read top to bottom, and the first one that matches wins, so order matters as much as content. Eight levels build up from a single ALLOW rule to full CIDR-based network segmentation. Your progress is saved locally.

NETWORK DEFENDERLVL 01
Rules
0
Default
ALLOW
Result
-

    ALLOW|DENY [TCP|UDP|ICMP] [port] [FROM cidr]: anything left out means "any".
    DEFAULT ALLOW or DEFAULT DENY sets what happens when nothing matches (ALLOW unless you say otherwise), so a bare DENY at the end closes everything you haven't explicitly opened. # starts a comment, for the rest of that line.

      INTERNETFIREWALLWEB SERVERVPNAPIDBUSERS

          Levels