How two browsers simultaneously listen on port 80?

Question

When I try to bind socket to port 80 from program I get error, but how two browsers simultaneously could listen to same port 80 ?

Answer 1

Browsers do not listen on port 80, HTTP servers do (although that's just convention, you could easily have an FTP or telnet server using port 80).

In TCP/IP, a "session" must be unique and the session is defined as the 5-tuple (protocol, sourceIP, sourcePort, destinationIP, destinationPort). This allows the packets to be routed correctly on the internet.

Typically when a client attempts to contact a server, it specifies 0 as its source port which means that the operating system assigns it an unused one. That means that the client will actually listen on that port rather than port 80.

So you may get a session with the properties (TCP, mybox.com, 1101, www.microsoft.com, 80) when your browser goes out to access Microsoft's web pages.

If you find you cannot bind your server to port 80, it will most likely because you already have a server running on that port, or your program doesn't have the required privileges to bind to that port (ports less than 1024 are generally considered privileged ports).

Running netstat -a (on Linux or Windows) will tell you whether a server is bound to port 80. Look for a listener on port 80 (or http if it's resolving ports to service names), something like:

tcp  0  0  localhost:http  *:*  LISTEN

Answer 2

They don't listen on port 80 they talk to port 80, or 443 if you're using SSL (or on any other port if the admin broke with convention, you may have seen urls like http://www.site.com:8080 where the site has been set up on port 8080).

The browser will make the request from a random high-numbered port so more than browser can be active at the same time.

As paxdiablo says, you can use netstat to see what programs are listening for connections (using "netstat -a -b" will show which executable is bound to which port)