How do tcpip ports work

Just as the IP address identifies the computer, The network port identifies the application or service running on the computer. The diagram below shows a computer to computer connection and identifies the IP addresses and ports. A port number uses 16 bits and so can therefore have a value from 0 to decimal. Port numbers — Well known ports. User written programs should not use these ports. Ports — These are used by client programs and you are free to use these in client programs.

When a Web browser connects to a web server the browser will allocate itself a port in this range. Also known as ephemeral ports. Client port numbers are dynamically assigned, and can be reused once the session is closed. TCP — is for connection orientated applications. It has built in error checking and will re transmit missing packets. UDP — is for connection less applications. It has no has built in error checking and will not re transmit missing packets. Applications are designed to use either the UDP or TCP transport layer protocol depending on the type of connection they require.

It can use any port, but the web server application is designed to use a TCP connection. Windows and Linux systems have a utility called netstat which will give you a list of open ports on your computer.

These articles show you how to use netstat on windows and on linux. You can check the port status of remote machines using a port scanner line nmap. It can be used with a graphical user interface of as a command line tool. Here is a useful article on using NMAP from the command line. Connection states — if you are wondering what established and listening and the other state descriptions mean. Online port tester Collection of tools for port scanning and web server testing.

I have a problem with an application that uses fixed source ports in the communication with the server. If the communication ends for some reason the clients tries to put the communication up again using the same source port. I have a software which creates 4 sockets with apparently random numbers the person who developed left no documentation. Is there a way to interconnect those random number sockets and port so the client is able to connect to the software?

Hi It sounds like the software is supposed to be a server which usually has fixed ports. What does the software do? It receives video data, connects to a MariaDB server, and serves as an API to it, receiving calls from a mobile app and sending customer data back. I wonder if the reason why it opens 4 sockets is to receive and send video data and receive and send customer data.

Either way, the problem is that it creates sockets of random numbers instead of a port which is what the android app uses. When you say it receives video data I assume that that is coming in on a port. The database would need another. The mobile app is used to view the video I assume. Is any part of it working. Where is the mobile App located internet or same network. Is this a standard kit or is it put together by yourself?

Rgds Steve. The software creates 4 sockets with random port numbers but none of them are the port which the APP uses. The mobile App is on the internet. The software connects to a tunnel connection I tried to use ssh -D Does it have any setup instructions.

Is it off the shelf software and does it have a website that I can take a look at. It is a bespoke software made by a person who left the company and never left any documentation. Hi, I have a question, is it possible for me to run 2 applications and connect to different ip but using same port number example ? Yes you are doing that when you connect to two different websites as they both use port 80 rgds steve.

So I have a question. Can server A somehow restrict me and only allow connections to the port on the remote server B and not allow me to connect me to any other port on remote server B? Have you seen such a restriction before and if yes then what would be the reason behind such a restriction?

Thank you. Not quite sure exactly what restriction you mean but you can restrict on destination port and ip and source port and IP if that helps rgds steve. You have a conversation and when it is over, you both hang up, releasing the connection. Using this method, the computer sending the data packages the information into a nice little package and releases it into the network with the hopes that it will get to the right place.

What this means is that UDP does not connect directly to the receiving computer like TCP does, but rather sends the data out and relies on the devices in between the sending computer and the receiving computer to get the data where it is supposed to go properly. This method of transmission does not provide any guarantee that the data you send will ever reach its destination. On the other hand, this method of transmission has a very low overhead and is therefore very popular to use for services that are not that important to work on the first try.

A comparison you can use for this method is the plain old US Postal Service. You place your mail in the mailbox and hope the Postal Service will get it to the proper location. Most of the time they do, but sometimes it gets lost along the way. Lets move on to the next section where we can describe the concept of ports better. As you know every computer or device on the Internet must have a unique number assigned to it called the IP address.

This IP address is used to recognize your particular computer out of the millions of other computers connected to the Internet. When information is sent over the Internet to your computer how does your computer accept that information? An easy way to understand ports is to imagine your IP address is a cable box and the ports are the different channels on that cable box.

The cable company knows how to send cable to your cable box based upon a unique serial number associated with that box IP Address , and then you receive the individual shows on different channels Ports. Ports work the same way. You have an IP address, and then many ports on that IP address.

When I say many, I mean many. When a program on your computer sends or receives data over the Internet it sends that data to an ip address and a specific port on the remote computer, and receives the data on a usually random port on its own computer. Note that once an application binds itself to a particular port, that port can not be used by any other application. It is first come, first served. This all probably still feels confusing to you, and there is nothing wrong with that, as this is a complicated concept to grasp.

Therefore, I will give you an example of how this works in real life so you can have a better understanding. We will use web servers in our example as you all know that a web server is a computer running an application that allows other computers to connect to it and retrieve the web pages stored there.

In order for a web server to accept connections from remote computers, such as yourself, it must bind the web server application to a local port. It will then use this port to listen for and accept connections from remote computers. You've probably heard that the world is moving from IPv4 to IPv6 due to address depletion. It's also entirely likely that the time will come when we'll have to expand the port range to accommodate additional services.

That said, the first 1, TCP ports are called well-known port numbers, and they're agreed upon among technology vendors. So if you and I were to go into business and sell a really nice FTP client software, we'd agree to work with the standard, well-known FTP port numbers.

A socket allows for a connection to another system that's already running some TCP server software. A socket takes a combination of an IP address and a port number. That means a single host can host multiple instances of the same service by using different port numbers. For instance, we can set up a web server that has "Site 1" listening on the default port of 80 and another web server. That is to say another website on the same server with the same IP address, "Site 2", but listening on Port One place is during server application configuration.

Which is also why working with your network administrator to allow for that traffic to flow on those port IDs are important.

Firewalls monitor ports to keep systems secure. Service addressing is another way to use port numbers. Once we install our enterprise application, we advertise the service using, generally speaking, a hostname and the port number. We wouldn't have to do that if it were a well-known port. If it's well known, we can leave it off. We use port numbers for troubleshooting purposes. Specifically, we can troubleshoot malware and identify rogue processes.

Firewall configuration often uses rules that denote both aspects of a socket. You might create allowances or traffic blocks based on IP addresses, port numbers, or both.

Regardless of your OS, you can always get to the netstat command line tool, although the specific parameters you use will depend on your OS. In Windows, start with a command prompt and type:. This will output a table of all current TCP connections on the system. Unfortunately, you can't do all that much besides looking at it.