What is RADIUS Server?
A RADIUS server is a central authentication service that answers one question for network equipment: "should this user be allowed in, and with what limits?" When a subscriber connects via PPPoE or hotspot, the router forwards the username and password to RADIUS, which checks its database and replies with accept or reject plus the user's speed, quota and session rules. It also records usage for billing.
The three functions โ Authentication, Authorization, Accounting โ are why ISPs of any size use RADIUS: user data lives in one place, not scattered across routers. An Indian WISP with ten tower routers keeps all 2,000 subscribers on one RADIUS database; a customer paying online is activated everywhere instantly. FreeRADIUS, the open-source standard, pairs naturally with MikroTik and a billing panel front-end.