What is a Firewall?

Any network connected to the internet gets its door handles rattled around the clock — automated scanners probe for open ports at your address exactly as they do everywhere else. The firewall is the doorman: it examines traffic against a rule set — which addresses, ports and protocols are allowed — and blocks the rest, in both directions. In practice a company runs several doormen at once: the router at the office perimeter as the first filter; the built-in firewall on every Windows and Mac, enabled by default and best left that way; dedicated appliances at firms with higher stakes, adding content inspection and category filtering; and — the confusingly named cousin — the WAF, or web application firewall, which guards not the office but the website: it sits in front of the web application and filters application-level attacks such as SQL injection and exploit attempts, with Cloudflare having made this layer affordable to everyone. Two truths worth internalizing as a business owner: a firewall is necessary but not sufficient, since phishing and weak passwords stroll in through the front door with an invitation; and firewall hygiene means everything not explicitly needed stays closed — every port opened "temporarily" for some vendor or aging application tends to stay unlocked for years.