   How the router know for who is the incomming message?
  I just get thinking on that concept..When servers send feedback to the ISP IP, how the router know for who to deliver the message?   
  errr.... through the IP address? The ISP is a registered domain, that gets converted into an IP address via DNS lookups, and the router can then route to that IP address.   ;  Good question...I think he's talking about what happens when your local network's router recieves a message from the outside world, how does it know where to deliver it. Port forwarding maybe?   ;  It either knows because it is directly connected to a machine, which, via ARP has told him "I own that address", or because someone programmed a static route for the subnet. Or, it might know because packets have been coming from that address on a given port during the last few minutes.In all other cases, it doesn't know, and will forward the packet on the default route, making it someone else's problem.   ;MAC Address?EDIT: This is better.   ;  If you're talking about NAT, then read my article on NAT punch-through (it's available in the Forum FAQ). It explains how a NAT "connection" is established in the router, and how the router knows how to return incoming packets using a look-up table.;  Hers the simple version, just for basic understanding not 100% technically correct:When a server sends information to a pubic IP (one thats on the internet, such as an ISP's IP) that is it's address.  Now the router will have an idea of where it goes, mainly due to it's routing table - a table that it generates by talking to other, nearby routers.The table will also contain Static Routes, which are defined by an administrator.And the most famous of Static Routes is the Default Route - which lives up to its name.  So its up to the detection abilities of the routers, and proper set-up of static routes by an Administrator for a router to work.If this all fails, the packet is dropped, but it's working quite well so far.Feel free to ask more questions.   ;  Hey, cant belive I forget that topic..Thanks for the answers, thats exactly what I wanted to know..I just got thinking, If me and my sister are on our computers, we both are behind the same router with the same ISP, we both are browsing on google waiting for a response, how the router knows what google response to deliver to us..I just stopped praticing winsock since I got behind the damn router, what a annoying little thing, before I could open 2 apps on my pc and send data to myself trough the ISP IP, works perfectly normal, awsome for praticing.Now that Im behind the router I cant find a way of doing this, doesnt matter if I send data to my own ISP( here is from where comes my question, if I send data to my own ISP, it will go to me? to my sis? apparently it doesnt go anywhere T_T), to my router lan IP, to the router default gateway, even if I send to the rollback IP(127.something), none of this works..frustrating. Kind annoying also that the book "Network Programming for MS Windows"(awsome book) doesnt cover anything about that.I got reading the nat punch-trough stuff sometimes, but it works just with a server help right? Its lots of stuff on that article, too much info, generally I give up on the half because I wasnt understanding a damn( conceptually I do understand a little, but I have no idea how It would work on code)   ;  if you know your public IP, then you can try port forwarding for your server app.127.0.0.1 should work though. your firewall maybe getting in the way.   