RADIUS became network login's standard by centralizing authentication

In the early 1990s ISPs faced a growing dial-in problem: every point of presence had to check usernames and passwords locally, so adding a user meant touching every access device and keeping credentials in sync everywhere.

In 1991 Livingston Enterprises proposed RADIUS and won Merit Network's contract: one server holds the user database and answers authentication requests from any Network Access Server, using a shared secret and compact attribute-value packets over UDP.

The IETF standardized RADIUS as RFC 2058 in 1997 and updated it as RFC 2865 in 2000, which notes the protocol is 'widely implemented and used'. Open and vendor-neutral, it spread through ISPs, universities and enterprises, and RADIUS remains the de facto standard for centralized Wi-Fi and VPN authentication.

One shared server checks credentials for every NAS

Centralizing credentials removed per-device configuration and drift.

An open IETF standard let every vendor implement it freely.

The shared-secret design worked over slow, lossy dial-up links.

Vendor-specific attributes let it grow beyond dial-up into Wi-Fi and VPN.

The standard that centralizes a recurring chore wins: by moving authentication out of each device into one server, RADIUS became the default for dial-up, Wi-Fi and VPN access.

RADIUS spawned an ecosystem of servers such as FreeRADIUS, Cisco ISE and Microsoft NPS, plus countless NAS and access-point clients; it still handles enterprise Wi-Fi and VPN authentication, with newer RFCs extending it for dynamic authorization and proxies.