Does TCP ports work only for their preassigned tasks or any port can be assigned any other tasks?

Question

There any 3-4 open TCP ports ( 135,139,445,12345) on any computer. I like to tranfer HTTP data thru any of them, How can I assign any of these ports to carry HTTP data. In general, I like to ask, is it possible to carry any data of my choice to any TCP ports

Answer 1

Yes, generally any TCP port can be used to carry arbitrary data or protocols (HTTP, FTP, etc.).

However, certain ports/port numbers are 'well-known' or registered (see Internet Assigned Numbers Authority) and running private protocols/applications on them may interfere other applications or devices on the network.

If you have full control over the source (the server), the intermediate network and the destination (the client), then you should have no trouble running HTTP over any port you choose.

On the other hand, if the intermediate TCP connection passes through several 'hops' or over the public Internet of which you have little/no control over - this may involve passing through firewalls, proxies or special routers. If so, it's possible that any of these devices may make certain assumptions about the port in use and not route your HTTP traffic properly.

For example, port 139 is the TCP/NetBIOS service, etc. Attempting to route HTTP traffic through port 139 over a firewall that blocks the NetBIOS protocol (commonly done to 'secure' internal LANs) would not work.

Answer 2

in theory, you can use any of the 65,536 tcp ports for any purpose you like

however, the purpose of a tcp port is to act as a "virtual connection" between 2 processes so in order to use a port other than 80 for http data you will need to have a website, or whatever the responding process is, configured to use the other port

to access a website using say port 12345 you would just put the port number at the end of the url like this:

http://www.yahoo.com:12345

but in this case it wouldn't work as the yahoo website os configured to respond only on port 80

Answer 3

there are some default ports
http - port 80
ftp - port 21

to specify using a different port you must specify so otherwise a default port will be used.

For example http://www.yourdomain.com:135
the colon represents a separator after which a port is specified. Of course the server must be configured to broadcast on that port.