What happens when a router runs out of port numbers?

Question

I noticed while looking at the site ipchicken (http://www.ipchicken.com/) that when I go on there from my desktop or my phone or the university wifi, it gives me a "Remote Port". There's a range of port numbers (it never seems to go above 65,000 or so). But what if I write a program that listens on every single port? Will the router run out of ports? Will it block new people from connecting? What happens when a router runs out of internal port numbers?

Answer 1

Ports in TCP or UDP protocols are stored in 16-bit integer, so it is only 65535 ports possible to use.

If you use every port on your computer, than any application which needs a socket (connection) will not have it. Functions like listen() or connect() will result in error until there will be free port to use.

If you use every port in router (with NAT) than every new connection will be buffered or dropped. If router is just a router, not a gateway with NAT, the problem with ports will not exist, because transport layer (4th layer in ISO/OSI model) is not analyzed.

Answer 2

"If no ports are available, the packet is dropped." http://www.cisco.com/c/en/us/support/docs/ip/network-address-translation-nat/26704-nat-faq-00.html

Answer 3

TCP/IP works as follows:

When you create a connection between client and server, it connects to a port based on port and ip address.

Your ip address is the ip address given by the server, and the port is anything given by the party that initiates the connection. For example, an FTP server will connect to port 21 unless the server specifically mentions to use a different port.

Setting up a connection can be done in 2 ways.

First, the server listens on the public port for any connections to initiate the link that is going to be made. Then one of the following two things happen:

1. The server creates a new socket using a different port and replies with: please connect to this port.
2. The server asks to which port on your side it can connect, and connects to that.

The difference here is that with the first, all connections are outbound from the client side, thus no ports needs to be forwarded on the router. Whereas the second item uses an incoming port that needs to be configured.

The difference in technique is basically a matter of bandwidth vs opening ports on the clientside.

Now, when from the outside, a connection is being made to your public IP, on any given port, your router will accept the port, and using the internal rules (routing) it will forward the communication to a client ip on the local network. If you are hosting a server, the connection will then be closed and resumed on a new port, freeing the public port.

As long as the port remains open, a new connection cannot be made, and thus the source will keep trying depending on the time-out settings until eventually it gives up, and a time-out occurs.

So basically what you want to do is creating rules for all ports on your router to forward data to your program, and let your program open the connection, and forward it to the new ip. This basically means you create a router yourself. As a result, your speed will slow down and if not done properly, connections may not terminate properly etc.

Basically you are creating a program that most likely the firewall/portforwarding rules on your router can do too.