521
521 is a non-standard Cloudflare status code meaning the origin server refused the connection.
- Odd
- Prime
Where 521 fits
Why 521 is special
521 is the most diagnostic of Cloudflare's invented codes: the origin is not accepting connections at all, usually because the service is down or a firewall is blocking the proxy's addresses. Unlike a generic 502 it tells you which end failed and how.
Not just a mathematical number
521is also used in ways where ordinary arithmetic doesn't apply — each listed below with what genuinely does and doesn't follow from it.
- Mathematical number
A genuine quantity — its mathematical properties (prime, square, factors, and so on) are real facts about it.
A genuine quantity that ordinary arithmetic applies to — it can be prime, even, a perfect square, added to another number meaningfully, and so on.
- Code
Also used as a code — HTTP status codes.
An assigned identifier from a fixed registry or standard — its digits don't represent an amount, and arithmetic on it is meaningless (HTTP status 404 + 1 is not a 'better' status; it's a different, unrelated code).
521 is part of
Factors & digit properties
- Divisors (2)
- 1, 521
- Prime factorisation
- 521
- Digit sum
- 8
- Digital root
- 8
Representations
- Decimal
- 521
- Binary
- 1000001001
- Octal
- 1011
- Hexadecimal
- 209
- Roman numeral
- DXXI
- Scientific notation
- 5.210 × 10^2
Other mathematical patterns
- Deficient number
- Lucas number
- Twin prime
Related numbers & connections
521 is Cloudflare's code for an origin server that actively refused the connection.
Numbers Around Us
- appears inHTTP status codes
why?
Strength 0.50, confidence "high".
- appears alongside520
why?
Strength 0.50, confidence "high".
Technology & computing
521 is Cloudflare's code for an origin server that actively refused the connection, which usually means the web server is down or a firewall is rejecting Cloudflare's addresses. Like the rest of the 52x range it appears in no RFC.
DocumentedTechnology & computing
Common questions
Is 521 a prime number?
Yes, 521 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 521?
The factors of 521 are 1, 521.
What is 521 in Roman numerals?
521 in Roman numerals is DXXI.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 1 reviewer-confirmed record, sourced and reviewed before publication.
Last reviewed: 3 August 2026