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How does a router track connections? When you send a http request it goes to the router which is then sent to another router, etc over the internet; but then how does it know to go to the server, is it port forwarded to a load balancer? Finally when the packet comes back how foes it know to go back to the pc because the packet was sent back to the router not the pc? Is it possible to establish a connection to a pc controlled by a router, from outside that router I.e. can you direct a packet to a pc not directly connected to the internet?

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You can pretty much bet any answer is going to link OSI Model. . . –  surfasb Jul 28 '11 at 3:25
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5 Answers

Routers connect networks via gateways or interfaces, simplistically:

Do I know where this packet should go?
  yes - is it one of mine?
    yes - send to appropriate interface.
    no  - send on to next gateway
  no  - return a packet saying unreachable

Your home router will run connection tracking software - it will keep a list of connections that the machines on your network have been talking to on the internet. Thus it will know the addresses and ports to return the responses.

TCP packets have a special field that can contain the address of the original machine. The router uses something called masquerading to insert its return address, and the destination machine also includes it. This can save a bit of work on the connection tracker, but also reveal internal ip addresses to the remote server.

Returning UDP packets on the other hand rely on the router's connection tracker module to return reponses to the original machine. It was a difficult problem to solve until a few years ago.

New incoming connections won't have any entries in the connection tracker, so the router won't know where to send it unless given specific forwarding instructions for that type of packet, and you'll find in all routers a way of being able to specify which machine on your network will receive new requests for port 80 for example.

TCP is a complex and amazing state-driven protocol, packets don't have to take the same routes, can be broken up into smaller pieces, arrive out of order, with error correction requesting individual packets to be resent, and the datastream can be reassembled with the applications at either end not being aware of any difficulties, unless they are too great to affect performance. Many PhDs have been earned through it!

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To understand routing, first you should have an understanding of the OSI model and you should also have an understanding of CIDR.

This is going to be fairly basic:

When a packet leaves a computer on a standard home network, the computer knows it is a private network and understands that the final destination is outside the network because of CIDR, and so sends the packet to the default gateway. The default gateway looks at the IP and sends it up the chain of routers until it reaches the target network (based on IP information in the packet header), that gateway will then forward it to the right IP on the network (sometimes this is based on the port in the packet details, sometimes just the IP, depending on the far end server/network). When the requested information is sent back, your router knows which computer it goes to because of a port that was chosen to mark the session and it knows which private IP that session belongs to. There is a whole lot more to it, but this is a fairly high level explanation to get the concept of routing. I'm sure others will correct me or add upon this.

What it comes down to for the answer for your question, yes, it is possible, you only need to know the public IP at the end point and the port that the connection needs to be made on. The router will be responsible for knowing which computer to forward that port onto to reach the server on the private network (based on NAT rules).

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So how does it work when two private pc's use the same port? Is this just very simplified explanation of sessions and sockets? –  Will03uk Jul 28 '11 at 3:02
    
If it is an outgoing connection on port 80, a session is created and the router will keep track of which private IP opened which session and forward the response appropriately. If it is an incoming connection to a server on a port behind a router, the router will forward based on NAT rules. –  MaQleod Jul 28 '11 at 3:18
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I'm not sure if I'm understanding this correctly but I'll give it my best guess.You asked "Is it possible to establish a connection to a pc controlled by a router?" I am assuming you are referring to remote access. You can use something like vnc or remote desktop or that. If you are talking about two pc's connected over the internet then check out VPN. If you mean one assigning an ip from another then that would be a server/client connection and over a WAN it would have to be something like a VPN.

The TCP packet contains the information to the LAN destination inside the packet.

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I'll clarified my question, thanks –  Will03uk Jul 28 '11 at 2:51
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Here is a basic overview of routing.

Basically, the first thing to realize is that there are various levels of routers, and routing.

The first bit of routing is really done by your PC, and its routing table. Part of the routing table includes a destination of 0.0.0.0, which is a catch all for anything it does not know how to specifically route itself, so it sends that traffic to the default gateway. The routing table keeps track of the local networks it is connected to, and for anything it does not know about, it uses that default gateway.

Next is your home router and cable modem. Both are similar in nature in that they are VERY basic. They really only know about the networks that are on either side of them. They also use routing tables, which are pretty simple, and also have a similar catch all route for anything they do not know about, so they can pass the traffic on.

Once the traffic gets to your ISP and beyond, you get into very complex routers, which have many interfaces to allow for multiple routes, and have VERY complex routing tables. These routers can cost $500,000 and up. They mostly use the BGP (Border Gateway Protocol) to keep their routing tables up-to-date dynamically, so they can find the shortest route, and if a particular router along that route goes down, they can re-route the traffic. They talk to other routers regularly to find out what paths are open, and update their routing tables based on that.

http://en.wikipedia.org/wiki/Border_Gateway_Protocol#Basic_update_processing

As far as how the traffic finds its way back, when you send out a data packet, there is a header that keeps track of where it came from, and where it is going to.

I am not sure what you were asking at the end.

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At the end I was just wondering if you can establish a TCP connection with a private pc that is not connected directly to the internet –  Will03uk Jul 28 '11 at 3:37
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What you're talking about here is not really routing, but network address translation as performed by a "firewall" or a router, combined with routing. The router/firewall is keeping a state table (mapping) of the source and destination addresses and ports of sessions that go through it. Normal routing does not do this translation and is stateless.

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