Let’s walk through some examples to see how the connection table and the
extended access control list work hand-in-hand. Suppose an attacker attempts to
send a malformed packet into the organization’s network by sending a datagram
with TCP source port 80 and with the ACK flag set. Further suppose that this packet
has source port number 12543 and source IP address 150.23.23.155. When this
packet reaches the firewall, the firewall checks the access control list in Table 8.7,
which indicates that the connection table must also be checked before permitting
this packet to enter the organization’s network. The firewall duly checks the connection
table, sees that this packet is not part of an ongoing TCP connection, and rejects