TCP Versus UDP
Germane to the theory behind firewalling is the idea that connections can be limited based on the service being offered or sought. Because each TCP/IP service has a unique socket number that it listens to (remember from the telephone analogy that a port is the extension number that the service person for the company answers), it’s reasonably easy for a router or proxy server to limit connections to these ports. Because limits are placed on the port numbers themselves, it follows that the service is also limited. This means that you can pick and choose among the services that might travel in or out of the firewall.
 | Actually, services ... |
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