127
127 is the largest signed 8-bit integer, and the first octet of the loopback address 127.0.0.1.
- Odd
- Prime
Where 127 fits
Why 127 is special
127 is two familiar limits at once. It is 2⁷ − 1, so a signed byte counts no higher, which is why so many old games capped stats and item counts there. It is also the whole 127.0.0.0/8 block reserved for loopback — every address in it means 'this machine', though 127.0.0.1 is the one anyone types.
Same number, different meanings
How 127 is understood across different contexts — never implying one meaning because of another.
- Mathematics
127 is two familiar limits at once. It is 2⁷ − 1, so a signed byte counts no higher, which is why so many old games capped stats and item counts there. It is also the whole 127.0.0.0/8 block reserved for loopback — every address in it means 'this machine', though 127.0.0.1 is the one anyone types.
- Technology & computing
127.0.0.1 is the loopback address, and the whole 127.0.0.0/8 block is reserved for it — over sixteen million addresses for a function needing one. Traffic sent there never reaches a network interface, which is why local development works with no connection at all.
- Science
A signed 8-bit integer spans −128 to 127, so 127 is the ceiling of a great many counters, limits and field widths in older software and in embedded systems where a byte is still the natural unit.
- Media & popular culture
127 Hours dramatises a documented survival in which the outcome was widely known before release. Reports of audiences fainting are notable precisely because the ending held no surprise.
127 is part of
Factors & digit properties
- Divisors (2)
- 1, 127
- Prime factorisation
- 127
- Digit sum
- 10
- Digital root
- 1
Representations
- Decimal
- 127
- Scientific notation
- 1.270 × 10^2
Roman numerals
C X X V I I
Binary
1 1 1 1 1 1 1
1 × 64 + 1 × 32 + 1 × 16 + 1 × 8 + 1 × 4 + 1 × 2 + 1 × 1 = 127
Octal
1 7 7
1 × 64 + 7 × 8 + 7 × 1 = 127
Hexadecimal
7 f
7 × 16 + f × 1 = 127
Read as powers of two: 64 + 32 + 16 + 8 + 4 + 2 + 1 = 127.
In historical numeral systems
Egyptian
1 coil of rope, 2 hobbles, 7 strokes
1 × 100 + 2 × 10 + 7 × 1 = 127. The signs are simply repeated and added; where they sit on the line is a matter of layout, not value.
Babylonian
2 vertical wedges (sixties place), 7 vertical wedges (units place)
2 × 60 + 7 = 127. Places are separated by a space on the tablet, with the higher place written first.
Greek (Ionic)
rho, then kappa, then zeta, marked as a numeral
rho-kappa-zeta + keraia
rho + kappa + zeta written in descending order and added — the letters carry their values wherever they stand, so this is addition, not place value.
Hebrew numerals
qof, then kaf, then zayin
qof-kaf-zayin
qof (100) + kaf (20) + zayin (7) = 127, written in descending order.
Maya
1 bar + 1 dot (upper, twenties place) over 1 bar + 2 dots (lower, units place)
6 × 20 + 7 = 127. Places stack vertically with the smallest at the bottom, so the upper group is worth twenty times the lower.
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
- Lucky number (mathematical sieve)
- Mersenne prime
Related numbers & connections
A local web server is reached at 127.0.0.1 on whichever port it binds, which is how development happens with no network at all.
View 80 →Technology & computing
127.0.0.1 is the loopback address, and the whole 127.0.0.0/8 block is reserved for it — over sixteen million addresses for a function needing one. Traffic sent there never reaches a network interface, which is why local development works with no connection at all.
DocumentedTechnology & computingGeography: International — IANA and IETFPeriod: 1981–The highest code point in 7-bit ASCII is 127.
DocumentedTechnology & computingGeography: worldwideThe entire 127.0.0.0/8 block — over sixteen million addresses — is reserved for loopback, and every one of them refers to the machine you are on. Only 127.0.0.1 is used in practice. The reservation is why a program can talk to itself over the network without any network existing.
DocumentedTechnology & computing
Science
A signed 8-bit integer spans −128 to 127, so 127 is the ceiling of a great many counters, limits and field widths in older software and in embedded systems where a byte is still the natural unit.
DocumentedScienceGeography: GlobalPeriod: 1960s–
Media & popular culture
127 Hours dramatises a documented survival in which the outcome was widely known before release. Reports of audiences fainting are notable precisely because the ending held no surprise.
DocumentedMedia & popular cultureGeography: United StatesPeriod: 2010
Mathematics
127 = 2⁷ − 1 is the fourth Mersenne prime, after 3, 7 and 31. It also has a rarer property: 2¹²⁷ − 1 is itself prime, making 127 a double Mersenne exponent. Édouard Lucas proved that 39-digit number prime in 1876 entirely by hand, and it remained the largest prime known to mathematics until 1951 — the longest any single prime has held the record since records were kept.
DocumentedMathematicsPeriod: Lucas's proof 1876; record held to 1951
Common questions
What is 127.0.0.1?
The loopback address — it means 'this machine'. The whole 127.0.0.0/8 block, over sixteen million addresses, is reserved for the purpose even though one would do. Traffic sent there never reaches a network interface, which is why local development works with no connection at all.
Why do old games stop at 127?
Because a signed 8-bit integer spans −128 to 127, so a byte-sized counter cannot go higher. It is the same reason unsigned byte limits stop at 255. 127 is also 2⁷ − 1, the fourth Mersenne prime.
Is 127 a prime number?
Yes, 127 is prime — it has exactly two positive divisors: 1 and itself.
What are the factors of 127?
The factors of 127 are 1, 127.
What is 127 in Roman numerals?
127 in Roman numerals is CXXVII.
About this page's data
Mathematical facts:computed directly from the number's value — nothing to source or verify externally.
Contextual records: 6 reviewer-confirmed records, sourced and reviewed before publication.
Last reviewed: 15 July 2026