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Gateways (Section 46.11) route packets from one network to another. Firewalls prevent some packets from being routed, based on a set of rules. Generally these rules are based on which direction the packet is going, to which port (Section 46.1) it is destined or from which port it came, which protocol the packet is using (TCP, UDP, or ICMP for low-level protocols, though sometimes firewalls also recognize higher-level protocols like HTTP), and so forth.
A fairly standard firewall ruleset would allow outgoing packets from all machines on the LAN, disallow incoming packets that weren't part of an established connection (which allows machines on the LAN to establish connections going out, but keeps outsiders from establishing incoming connections), and then specifically allow things like incoming connections to port 25 (the SMTP (Section 46.8) port) on the mail server machine, ports 80 and 443 (the HTTP and HTTPS ports) on the web server machine, and port 22 (the SSH (Section 46.6) port) on any server that should be able to receive SSH logins.
Cable modems and DSL routers generally have simple firewalls built in; a Unix machine functioning as a gateway can also firewall and often has much more complex capabilities. Firewall software varies enough that detailed configuration of a firewall is beyond the scope of this book; things to look for include the documentation for ipfw, ipchains (Linux 2.2 kernel), or iptables (Linux 2.4 kernel).
-- DJPH
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| 46.11. Gateways and NAT |  | 46.13. Gatewaying from a Personal LAN over a Modem |
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