The Fire Wall is designed to let in only those packets that meet its criteria. This wall is operating on ports 80 and 25. All attempts to enter through other ports are closed for business.
Port 25 is used for mail packets, while port 80 is the entrance for packets from the Internet to the Web server.
Inside the fire wall, packets are screened more thoroughly, some packets make easily through "customs" while others look just a bit dubious. The Fire Wall officer is not easily fooled, such as when this ping of death packet tried to disguise itself as a "normal" ping packet.