In the preceding discussion, the firewall forwards everything except
where specifically instructed to filter out certain kinds of packets. A firewall
could also filter out everything unless explicitly instructed to forward
it, or use a mix of the two strategies. For example, instead of blocking
access to port 80 on host 128.7.6.5, the firewall might be instructed to only
allow access to port 25 (the SMTP mail port) on a particular mail server,
such as