[java] Basic networking problem

It's possible because you are connecting from the same machine. Port number only matters if you have something like a NAT firewall that blocks certain port numbers. If you have nothing like this all your ports are open. I also think that if you connect to yourself its the same as doing a 127.0.0.1 connect which the any firewall does not detect.

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Good morning

Well, good evening to you (actually its midnight) ;-)

what you have to consider is that there are in fact 2 ports you are working with, the outgoing port and the incoming port.

The outgoing port can be anything (and yes, the OS will decide which port to use dynamically), the incoming port must be specified, That is unless you need the output port to be something specific.

Next 256 converts to 0.0.1.1 because the values of each IP octet are from 0 to 255, so 255 will convert to 0.0.0.255, in hexidecimal 0x000000FF

Additionally, to get an IP address from a string you can use the inet_addr(char*) function

To create a connection to the same computer your outgoing connection is on, you need to set the destination IP as 127.0.0.1

Good morning,

I couldn't get it work so I switched to using TCP.

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ZedFx Next 256 converts to 0.0.1.1 because the values of each IP octet are from 0 to 255, so 255 will convert to 0.0.0.255, in hexidecimal 0x000000FF

That wasn't what I asked about.

If I specify an erroneus host by its name (e.g. myPC), it will not connect. However, if I specify a host by its numeric IP (e.g. 0.0.1.1 or 210.1.3.4) it will connect! How is that possible?

Note that my pc's IP is 192.168.1.101 and I am running two firewalls, Windows' built-in firewall as well as zonealarm. Also, I connect to the Internet through a wireless router, though I don't think this is of any importance.

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Original post by bah Good morning, I couldn't get it work so I switched to using TCP. Quote: ZedFx Next 256 converts to 0.0.1.1 because the values of each IP octet are from 0 to 255, so 255 will convert to 0.0.0.255, in hexidecimal 0x000000FF That wasn't what I asked about. If I specify an erroneus host by its name (e.g. myPC), it will not connect. However, if I specify a host by its numeric IP (e.g. 0.0.1.1 or 210.1.3.4) it will connect! How is that possible? Note that my pc's IP is 192.168.1.101 and I am running two firewalls, Windows' built-in firewall as well as zonealarm. Also, I connect to the Internet through a wireless router, though I don't think this is of any importance.

Note that specifying a host name requires you to be running some sort of DNS server running on a machine that you tell your home machines to look at for DNS lookups (the thing that happens when you type in "MyPC" instead of it's ip), or have the entry of the machine with ip and host name in a "hosts" file.

As for your machine allowing your machine to connect to any port, i.e. 127.0.0.1 requesting any port on 127.0.0.1 would be in a rules set somewhere in a firewall that allows all connections from "localhost/127.0.0.1." You should try to connect to the machine that is the server from a different one on various ports to see if you can still connect, and if you can still connect to any arbitrary port on the server, you may want to consider checking your firewalls' settings since having open ports is a huge security risk.

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Original post by garazdawi It's possible because you are connecting from the same machine. Port number only matters if you have something like a NAT firewall that blocks certain port numbers. If you have nothing like this all your ports are open. I also think that if you connect to yourself its the same as doing a 127.0.0.1 connect which the any firewall does not detect.

There is a huge difference between NAT and firewall. In short a Firewall is a piece of hardware or software that blocks incoming network traffic on non established connections (i.e. connections that wern't initiated from behind the firewall, and thus becoming established.) NAT is Network Address Translation which in short is port forwarding. Say you have a machine (192.168.1.2) behind your firewall/router that has NAT that accepts connections on port 1111. On your firewall/router you can set it up to accept connections to the firewall/router on port 5000 and forward the connection to machine 192.168.1.2 on port 1111. So then you can connect to your firewall/router's ip (say 192.168.1.1) on port 5000 and it would be the same as you connecting to 192.168.1.2 on port 1111.

Sorry if I was confusing above, I ment a NAT router with a port blocking firewall.... that day I was destined to be confusing :o