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I have my internal network: 192.168.0.0/24

My router is assigned the external ip: 231.123.11.12

My question is, if from one of my internal computers (192.168.0.100), I connect to the external IP, and transfer a large file, will this usage count towards my ISP bandwidth usage?

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These answers are correct in that the file will not count towards ISP usage since it will not go to your ISPs network. But what's the point of sending a file to your router? What are you really trying to accomplish? The question as worded doesn't make much sense. –  cb4 Jun 6 '13 at 4:50
    
@cb4 I run a local media server, and have a domain which points to my external IP. My router forwards certain ports to the media server. I was wondering if connecting to the media server through the domain name, would route traffic over the internet. –  Petey B Jun 7 '13 at 16:37

2 Answers 2

up vote 1 down vote accepted

When your router sees a packet destined for one of its interfaces, it will not forward the packet to your ISP.

Here's a little more detail: Your router uses a routing table. The router looks at the destination IP on a packet and uses the table to look up where to send it. If there is a specific entry for that IP or for a subnetwork containing that IP, it will use that interface to send the packet.

There is a special route called "default" that matches any other packet. Typically the only packets that go to your ISP are ones that match the default route. In your case above, the router sees that it has an interface that matches your destination IP (its own interface) and delivers it there (to itself).

Depending on how your network is set up, however, this may not result in the best way to transfer a file.

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It should not count as it doesn't enter your ISP's network (it never leaves yours), however, if your router doesn't support hairpin routing you may not be able to connect (something that many routers still don't seem to support well, if at all).

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