ports and NAT

ok, so i've been kind of curious as to what a port is. i still don't really know, but i think it's kind of like an address for a specific function within a computer. a computer has lots of ports. they're not physical things, more like indices for input and output.

anyway, i was reading about network address translation (NAT), and a part of understanding it requires the concept of ports. NAT is where a computer locally has one IP address, but to the rest of the internet it appears to have a different IP address, and possibly the same address as lots of other computers that are on the same local network. this happens because they're all on a private network, say, and they're all using a router to send info out into the internet, and get info back out of it. the router knows all of the computers on the private network by their private IP addresses, and it assigns each of these to a specific port number for its own IP address (the router being just another computer in the network).

so, when a computer on the private network sends a message out into the internet, its private IP address gets changed ('translated') into the IP address of the router plus a specific port number. incoming messages meant for that computer must have the correct port number; basically, for the router, port numbers refer to computers on the private network.

but that's not enough, because each of those computers is using different ports to do different jobs with different targets on the network: one port keeps in touch with the Skype supernode, one port is getting data for a file i'm downloading, and another port is sending the info that i'm typing into this blogger.com window right now. so, actually, the router has to assign a different port number to each port on each computer on the private network; so, for the router, a specific port number will refer to a specific port on a specific computer on the private network.

i'm pretty sure this is all true for the protocols that have to do with sending and receiving files. i still need to learn about protocols, but i think there are also protocols for sending packets to all computers on a network, so maybe you wouldn't need to know their port numbers exactly to do that. not sure.

anyways, there's some stuff about ports.