What is NAT

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What is NAT, what is port forwarding and why do I need to know all this stuff just to make Azureus work?[edit]

Imagine this: a phone rings at your company's head office, and the caller asks to be put through to Extension 6881. The receptionist has never heard of Extension 6881 (although a lot of callers have been asking for it lately) and has been warned about Nigerian scamsters, so she just hangs up.

Meanwhile, in another building, Mr. Azureus is making conference calls. He's called everybody he has business cards for, and is having a fine time talking with them all; but he knows he could get at least twice as much done if he could talk to more people - if only somebody would call HIM for a change!

Mr. Azureus complains to the personnel manager (that's you) that the boss is always at him to work faster, but he can't because even though he has given out a heap of business cards with the company's phone number and his office's extension, nobody ever calls him and the pressure is making him jaundiced.

Mr. Azureus wishes he could just put his own building's phone number on his business cards, but he can't; that's a private number which is only meaningful to the company's internal phone system.

So you call head office to sort things out. You identify yourself as the personnel manager and tell the receptionist she should put all calls for Extension 6881 through to Mr. Azureus's building, and you give her the building's private phone number.

Now Mr. Azureus's phone is ringing all the time, and he's happy because he can get enough done to please his boss, and is pretty sure he's due for a raise instead of getting offshored.

NAT is Network Address Translation, and it makes it appear from outside as if your entire LAN is a single PC accessible via your NAT router's public IP address. To machines out on the Internet, it looks as if Azureus is running on your NAT router, just as people calling head office don't perceive that Mr. Azureus works in a different building.

So you need to tell your NAT router how to forward Mr. Azureus's calls, or he won't get any. You also need to tell Mr. Azureus what your router's public IP address is, so he doesn't put a meaningless number on his business cards.

  • Network hosts (PC, router etc) are like buildings
  • IP addresses are like phone numbers for buildings
  • Port numbers are like extension numbers for offices inside buildings
  • Azureus is like a worker in an office
  • Your LAN is like an internal company phone system
  • Private IP addresses are like phone numbers meaningful only to the internal company phone system
  • The Internet is like the global phone system
  • Public IP addresses are like phone numbers meaningful to both the global and internal company phone systems
  • a NAT router is like head office; it's the only host on your LAN with a public IP address


Read the Azureus FAQ