In firewalls, the technology used most often to control traffic. The fields in every packet are compared against a rule set configured on the firewall. Rules might be of the following form:
So, if the private network is 196.0.3.x, the initial rule in the preceding list blocks all incoming TCP connections, but outbound connections can continue. The following rules override the first; thus, access to the email server at port 25 is allowed and access to the Web server at port 80 also is allowed.
Packet filters are susceptible to fragmentation attacks, whereby an attacker splits up a TCP connection into many smaller packets to avoid detection by packet-filtering rules.