Firewalls and Proxies

A router sits between two networks and routes packets between them by looking at the destination IP address. It knows which numbers are "left" and which are "right".
A firewall is like a router but it can decide not to route certain packets. It often looks at the port when making a decision (e.g., block all telnet connections).
- Most companies place a firewall between them and the Internet.
A proxy is a go-between. Outside machines don't see the IP of machines inside. It works at the application level. e.g., Internet Object cache.
In a client-server architecture the server provides services which the client consumes (e.g., browser-webserver).
An n-tier architecture expands this concept to multiple levels (e.g., browser-webserver-database).