Final answer:
To block all external websites, Bart should block port 80 and port 443 on the firewall, which handle HTTP and HTTPS traffic, respectively.
Step-by-step explanation:
If Bart wants to block access to all external web sites, he should primarily block port 80 and port 443 at the firewall. These ports are commonly used for HTTP and HTTPS traffic, respectively, which are the protocols used by web browsers to access websites.
Blocking these ports will prevent users on the network from accessing websites using standard web protocols. However, it's important to note that this might not block all web traffic, as some services may use different ports or methods to connect to the internet. Furthermore, blocking these ports can affect all outbound web traffic, so it would be necessary to consider the impact on legitimate business or educational functions before implementing such a block.