80

A browser omits :80 from the address bar because it is the scheme's default. Typing it explicitly produces the same page, and its absence is why most users have never seen a port number despite using one every time they load a page.

Ports below 1024 are the well-known range administered centrally, originally by Jon Postel and now by IANA. Port 80 was assigned to HTTP in 1990 while the protocol was still a proposal, which is why the web's default predates the web's popularity.

Port 80 was HTTP's default from the beginning, and for most of the web's life it carried nearly all of it. Traffic has since moved almost entirely to 443 as encryption became the default, so port 80's main remaining job is redirecting visitors to the encrypted version of the same address.

The IBM punched card held 80 characters, terminals inherited the width, and coding styles still argue about it. It is one of the clearest cases of a physical constraint from the 1920s outliving the object that imposed it by a century.

'Around the World in Eighty Days' — Jules Verne's 1873 novel — made 80 a recognisable adventure-story number, referring to Phileas Fogg's wagered round-the-world journey.

80 is the atomic number of mercury, the only metal liquid at room temperature. Its steady expansion made it the working fluid of thermometers and barometers for centuries, until its toxicity ended most such uses.