How ADSL connections work through router

Question

I have a ADSL broadband DHCP connection. I am using D-Link GLB-802C ADSL router. Now I want to know what actually happens when I access internet through browser or any other program. I know all http communication is handled via TCP. I have following questions.

• while setting up the internet with the disk provided by router, I didn't made any setting of proxy so is the proxy not present, I just entered the username and password
• If router handling connection to ISP proxy, Do I need to make special configuration for JAVA P2P applications to communicate with other peers.
• My internet is working fine and when I checked network settings of Java they are configured to use browser settings.Are there any other settings required

I would also like to know do I need to forward a port for all P2P applications. I am currently trying JXTA and facing lot of problems to make it work over internet. I did started the thread here http://stackoverflow.com/questions/1371549/jxta-p2p-application-over-internet but not getting any answers so I am digging into this to make it work.

Answer 1

Routers use NAT protocol to handle that. Making the long story short, routers relay TCP/IP packets directly between your computer and the destination server. There is no need to use proxies, the important setting is called ‘default gateway’ or ‘default route’.

But that method works that easy only for outgoing connections (as your router knows which computer from internal network has initiated the connection). When an incoming connection comes, it has only your external (router) IP and router can't know to which computer the packet is designated.

And there's where port forwarding comes. By setting it, you tell router that your computer is going to handle connections to that specific port, and all incoming connections to it should be relayed to your machine.

So, the answer may be a little ambiguous as it depends on how specific P2P protocol works. In most cases, you don't need to have the port forwarded as long as your peer has one (or external IP), or some proxy is used to relay your connections.

But I think it's a good idea to always have a port forwarded if it's possible as if you're going to use that on an Internet-wide basis, you can't guarantee that your peer will be able to forward him-/herself a port.