821
RFC 821 defined the Simple Mail Transfer Protocol.
- Odd
- Prime
Where 821 fits
Why 821 is special
Its lack of sender authentication is why spam and forged email are structurally possible, and why SPF, DKIM and DMARC all had to be bolted on afterwards.
821 is part of
Factors & digit properties
- Divisors (2)
- 1, 821
- Prime factorisation
- 821
- Digit sum
- 11
- Digital root
- 2
Representations
- Decimal
- 821
- Scientific notation
- 8.210 × 10^2
Roman numerals
D C C C X X I
Binary
1 1 0 0 1 1 0 1 0 1
1 × 512 + 1 × 256 + 0 × 128 + 0 × 64 + 1 × 32 + 1 × 16 + 0 × 8 + 1 × 4 + 0 × 2 + 1 × 1 = 821
Octal
1 4 6 5
1 × 512 + 4 × 64 + 6 × 8 + 5 × 1 = 821
Hexadecimal
3 3 5
3 × 256 + 3 × 16 + 5 × 1 = 821
Read as powers of two: 512 + 256 + 32 + 16 + 4 + 1 = 821.
In historical numeral systems
Egyptian
8 coils of rope, 2 hobbles, 1 stroke
8 × 100 + 2 × 10 + 1 × 1 = 821. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
1 corner wedge + 3 vertical wedges (sixties place), 4 corner wedges + 1 vertical wedge (units place)
13 × 60 + 41 = 821. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
omega, then kappa, then alpha, marked as a numeral
omega-kappa-alpha + keraia
omega + kappa + alpha written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
tav, then tav, then kaf, then alef
tav-tav-kaf-alef
tav (400) + tav (400) + kaf (20) + alef (1) = 821, written in descending order.
Systems that simply cannot reach this value are left out. Where a script needs a font most readers do not have, the reading is given instead of the glyphs — see number systems for each system in full.
Other mathematical patterns
- Deficient number
- Twin prime
Related numbers & connections
RFC 5321 replaced RFC 821 after twenty-six years while keeping the permissive envelope, because changing it would break every mail server simultaneously.
View 5321 →Technology & computing
RFC 821 specifies how mail is relayed between hosts. It assumes cooperating operators and provides no way to verify that a sender is who the envelope claims, which later anti-forgery mechanisms address externally.
DocumentedTechnology & computingGeography: International — IETFPeriod: 1982
Common questions
Is 821 a prime number?
Yes, 821 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 821?
The factors of 821 are 1, 821.
What is 821 in Roman numerals?
821 in Roman numerals is DCCCXXI.
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: 15 July 2026