Port Authority Edition – Internet Vulnerability Profiling
by Steve Gibson,  Gibson Research Corporation.

Goto Port 80
Probe Port 81
Enter Port: 0-65535
Goto Port 82

Port Authority Database

Port 81


HOSTS2 Name Server

We have no idea what this port's historical use was, and neither, apparently, does anyone else. It is listed (as shown above) on everyone's "port list", but no further information is ever offered.

Its current use, and its importance today, is due to is adjacency to the super-popular "http" world wide web port 80. Sometimes an "off the beaten path" web server will be established to listen not to port 80, but to an alternate related port such as 81, 82, 8080, or 8090. (Related by proximity or appearance.) So port 81 is only noteworthy today because it is a neighbor to port 80 and is sometimes used, as an adjunct or alternative, in the same way.

Related Ports: 
80, 82, 443, 8080, 8090

Background and Additional Information:

URL Defaults: Everyone today is familiar with the unfortunately awkward nomenclature used by the world wide web. The fact that we are all burdened by the ubiquitous, redundant and largely unnecessary "http://www." prefix is an unfortunate accident of the Internet's evolution. If we had it to do over again, knowing how popular and important this one protocol (http - Hyper Text Transfer Protocol) and use (www - World Wide Web) would become, we would certainly have done things differently so that the baggage of their explicit and sometimes confusing specification wouldn't haunt every use and user of the Internet.

As you can see by the URL showing for this page (in the URL Address field above), and probably through your own experience, the "www." portion of the "machine name" is often optional or implied. But sometimes it isn't, and a "www." must be added to the site's domain name. Other sites work without it, but not with it. It's a mess. (And just TRY explaining this to someone who doesn't like computers and is desperately trying to avoid learning anything about them.) In a similar fashion, since http is the default protocol used by web browsers, users are often saved the burden of entering the initial "http://". In the absence of anything explicit, many browsers safely assume or provide it.

Specifying an alternate port: However, as we explained on our page for port 80, port 80 is the "default port" for the web's http protocol. How, then, if we wish to use an "alternate port" like 81 or 8080, do we override this default to specify the port we desire?


If we wish to override the default service port that is implied by the protocol (http), we place a colon (:) after the domain name portion followed by the port we wish to use. The URL shown above would cause the browser to override its normal use of port 80 for http, and instead attempt to connect to a server listening at "www.domain.com" for connections on port 81.

If you ever see a URL using this format, with some number following a colon after the URL's domain, you'll know that this URL is asking the browser to connect to a server listening for incoming connections on the specified port rather than its default.

Trojan Sightings: RemoConChubo

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