What does RADIUS stand for?
RADIUS stands for Remote Authentication Dial-in User Service. An AAA protocol that lets network devices such as wireless controllers, VPNs and switches check user credentials against a central server. UDP ports 1812 (authentication) and 1813 (accounting); it is the back end for WPA-Enterprise and 802.1X.
RADIUS centralizes authentication, authorization and accounting for network access. A switch, wireless controller or VPN gateway acts as the client and forwards the user's credentials to the RADIUS server, which answers accept or reject. It uses UDP, with port 1812 for authentication and 1813 for accounting, and it protects the password field while leaving the rest of the packet readable.
RADIUS in practice: an example
On an 802.1X wireless network, a laptop presents its credentials to the access point. The access point does not decide anything: it passes the request to the RADIUS server, which checks the directory and replies Access-Accept with the VLAN the user should land in.
What RADIUS is confused with
RADIUS vs TACACS+: what is the difference?
RADIUS uses UDP, combines authentication and authorization and encrypts only the password; it is the usual choice for network access. TACACS+ uses TCP 49, separates the three AAA functions and encrypts the whole payload; it is the usual choice for administering network devices.
Which exams cover RADIUS?
RADIUS is on the official CompTIA acronym list for two of the four exams covered here.
- Network+ N10-009: 4.0 (Network Security).
- A+ 220-1201 and 220-1202: 220-1202 2.0 (Security). Listed there as “Remote Authentication Dial-in User Server”.
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. A company wants each employee to join the Wi-Fi with a personal account instead of a shared passphrase. What does the wireless controller need to talk to?
- A RADIUS server
- A DHCP relay
- An NTP server
- A syslog collector
Show the answer
A. A RADIUS server. WPA2 or WPA3 Enterprise uses 802.1X, and the controller forwards each authentication to a RADIUS server.
2. A firewall between the access points and the RADIUS server allows only TCP. Why does authentication fail?
- RADIUS uses UDP 1812 and 1813
- RADIUS needs ICMP only
- RADIUS uses TCP 49
- RADIUS requires IPv6
Show the answer
A. RADIUS uses UDP 1812 and 1813. RADIUS runs over UDP. TCP 49 is TACACS+.
Sources: RFC 2865: Remote Authentication Dial In User Service (RADIUS). The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.