C# – How to connect computers via IPAddress

cnetworkingsockets

This is related to c# Sockets.
I'm developping a socket program. But there is one problem in my software.
I cannot connect to computers which are connected to internet via theirs local network. But I can connect to computers that are uses internet alone. Be more descriptive,
Consider that 5 computers connects internet via a modem that uses 1 ip address. When I try to reach one of these computers, API connects to modem via its ip address. But there is no response. Because a modem don't respond a computer request. By the mean, my API must reach a computer not only modem. And and an SocketException is thrown. What can I do about this problem?

Best Answer

The problem you're encountering is caused by NAT. This is used by routers who allow multiple clients to go online through one public IP address. None of the clients behind the router will 'know' or 'see' this, but it limits connectivity.

Clients can initiate a connection to the outside, but the other way around is basically* impossible. Unless you use port forwarding, where one or more ports are forwarded to a client behind the router. This way, connections can be initiated from the outside.

This requires configuration on the client side however, so the preferred way would be to let your clients connect to your server, since that will always be possible (firewalls neglected).

*: There also is a workaround, where you let a client connect to your server, and then pass the connection information to another client, so those clients can communicate with each other. This is called 'nat punching' and is used by torrent clients, for example.

New answer, powershell

TCP

Get-Process -Id (Get-NetTCPConnection -LocalPort YourPortNumberHere).OwningProcess

UDP

Get-Process -Id (Get-NetUDPEndpoint -LocalPort YourPortNumberHere).OwningProcess

Old answer, cmd

 C:\> netstat -a -b

(Add -n to stop it trying to resolve hostnames, which will make it a lot faster.)

Note Dane's recommendation for TCPView. It looks very useful!

-a Displays all connections and listening ports.

-b Displays the executable involved in creating each connection or listening port. In some cases well-known executables host multiple independent components, and in these cases the sequence of components involved in creating the connection or listening port is displayed. In this case the executable name is in [] at the bottom, on top is the component it called, and so forth until TCP/IP was reached. Note that this option can be time-consuming and will fail unless you have sufficient permissions.

-n Displays addresses and port numbers in numerical form.

-o Displays the owning process ID associated with each connection.

Best Answer

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New answer, powershell

TCP

UDP

Old answer, cmd

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