As in a telephone call, there are two roles you can play in order to establish a connection: Caller Receiver The short version is as follows: Receiver: fill in the Local IP and Local port fields and click the "receiver" button; upon incoming call click "answer" on the popup dialog to accept. Caller: fill in the Public IP and Public port fields and click the "call" button. The tough part, however, is the Receiver, as it may have to deal with network configuration. Or (since version 1.1) you can use UDP hole punching. Since this is probably the main criticism in peer-to-peer applications, and we can rely only on ourselves (remember: no server in the middle!), I'll try to put it very simple in the next paragraphs. Contents of this section: Network configuration Receiver Caller UDP hole punching

Think of ViaVoip as your home telephone. To get called you'd give your phone-number, don't you? Well in computer networks a "phone-number" is a pair value made up of:

• IP address or just IP: uniquely identify a computer (or any other network-enabled device) in a network;
• port number or just port: uniquely identify a program in a computer.

All programs that need a network connections must have a port assigned.
Think as IP was the company-number, and port the extension.
IP addresses are usually written and displayed in human-readable notations, such as 192.168.10.1 (each of the 4 dot separated numbers can range from 0 to 255).
Port numbers can range from 0 to 65535, but usually 0 to 1023 are reserved by your operative system and you can use from 1024 up.
The IP and port pair is usually written consecutively, separated by a colon, such as 192.168.10.1:1024.
Here is a working example on a LAN (Local Area Network), i.e. your home or office network:

192.168.10.1:1024 <-> 192.168.10.2:1024

So far so good: your Local IP and Local port is all your peer needs to know to call you if she's inside your LAN.

To go on with the phone example, if your peer lived outside your town than she would prefix your number with your area code, right?
If you connect to the internet through a router/gateway and your peer is outside your local network, she's considered out of town; and what is the area-code prefix equivalent in a computer network? The router Public IP address.

You got it: routers have two different IP's, and you can only see the one on your side of the network.

• Public IP: this is the only IP visible outside your local network
• Private or Local IP: this is your router IP inside your local network.

So, to get called, you'd just give your Public IP address and your Local IP and Local port.
But unfortunately, everything is inside your local network is unreachable from the outside.
And here's where NAT (Network Address Translation) comes into action:
routers have port numbers as well, and you can map a router Public Port to an Local IP:port pair in your local network, thus letting your peer think calling you at Public IP:port, but in fact forwarding it to your Local IP:port.

192.168.0.10:1024 <- 85.10.58.63:1024 <-> 72.248.26.116:1024 -> 192.168.1.100:1024

That is sort of unbalanced, one (router) against n (computers): the router has port range from 0 to 65535, as each of any computer in the LAN, if all needs a few thousands ports mapped, the router could easily run out of available ports...Correct, but very unlikely though: in real life only very few ports needs to be mapped, just those that must accept incoming connections. For example? Emule, Torrent and...ViaVoip (only when it wants to run as Receiver).

So, let's see now how to configure NAT.

NAT configuration varies from router to router, but basically all you have to do is open your browser and navigate to your router administration page. Here is a real life example:
on the first line you can see I have forwarded my routers Public port 1024 to my laptop at Local IP 192.168.1.104 on Local port 1024;
Both TCP and UDP protocol must be forwarded to the same port




Guessed it: my router Local IP is 192.168.1.1. That is exactly what I put on my browser address bar. To find out what is yours, open up a terminal:

• Windows: goto Start menu -> run and type 'cmd';
• Mac OS X: open Finder and go to Applications/Utilities and run 'Terminal';
• Linux: you should now!
• Android: I don't know!

and type:

• Linux & Mac OS X: ip route or route -n
• Windows: ipconfig

Here is what number you need to pick up on windows:

Finally, to know your Public IP address there's no way but asking an external service for reporting it back to you. There are a lots of sites offering it, but to avoid you this pain ViaVoip does it for you, querying this Public IP discovery service and filling up the Receiver's "Public IP" field with the response.
We'll see this in more details in the next paragraph (feut!).

• Configure your NAT port mapping if needed (see previous paragraph);
• Select an internal IP from the Local IP combo:
  More than one IP can be present depending on your hardware configuration (i.e. you can have both a LAN and a WI-FI network card active), choose the one you've mapped in your NAT configuration.
  the IP "127.0.0.1" is the local host "loopback" address, and is not visible by any device on the network, it is there only for testing purposes.
• Set the local port you want to receive incoming calls in (the one you forwarded in your NAT, if needed)
• Click on the Receiver button in the top tool-bar: if everything is ok the button icon turns green.
• Let your peer know your pubblic IP and Public Port the first time she calls you.
  Public IP's rarely change, but if your Public IP has changed since last time you connected the refresh button icon will turn into an alert sign (hover mouse on to read message).
  (NB: the (:xxxxx) port number visible in the public IP field is the port your router let you through querying the Public IP discovery service, and not that to connect to your peer).
• Upon an incoming call arrival this popup dialog will show up:
  
  
  
  Just click answer and line is on!
  Notes: the "name" and "last call" field are explained in the "history" section; audio input and output are automatically started.
• When connected the Receiver is paused and will automatically be restarted upon hung-up.
  

Just fill in the Public IP and Public port fields as instructed by your Receiver peer and click on the Call button.

The (Local) field shows your current local end point (ip:port) (if you are the caller you don't choose your local port, but the system picks this one up for you).

Below the (local) field you can see starting from the left:

• outgoing bit rate / total bytes sent;
• call duration;
• incoming bit rate / total bytes received;

So what if you can't (or don't feel like) configuring the router you connect through?
Whe can exploit the routers algorithm that, whenever you make an outgoing request (i.e. browsing a remote web page) maps dynamically a random port to the LAN endpoint (IP:port) that made the request, and sends it back any incoming response. This technique is better implemented over the UDP protocol.

When Bob wants to receive data from Alice, he sends any data to her remote port, even if Alice's firewall will reject this data (left picture).
But if straight after Alice sends some data to Bob's remote port, Bob's router will think this is the response coming from Alice, and will let her in (right picture)!
As there is a timing issue (i.e. Alice's data reach Bob's router before Bob's has sent any data) this can imply a few try from both sides.
The same apply viceversa, so Bob can also send data to Alice.
And with the same mechanism, after a few try, Alice will be able to send and receive data from Bob.

then choose two local ports and click the refresh button

and tell your peer what you read in the "Public IP" field [IP:port1,port2] by any other means (you can copy/paste the full text in a message). Then fill in the equivalent fields in the caller section with your peer's info (you can paste the full [IP:port1,port2] text via the "paste" button)

Now click the Call button and wait...good luck!

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