Paywall
A paywall is a method of restricting access to content via a paid subscription. Beginning in the mid-2010s, newspapers started implementing paywalls on their websites as a way to increase revenue after years of decline in paid print readership and advertising revenue. In academics, research papers are often subject to a paywall and are available via academic libraries that subscribe.
Firewall (computing)
Commonly used packet filters on various versions of Unix are "ipfw" (FreeBSD, Mac OS X (< 10.7)), "NPF" (NetBSD), "PF" (Mac OS X (> 10.4), OpenBSD, and some other BSDs), "iptables"/"ipchains" (Linux) and "IPFilter".