Difference Between Public and Private Ports?

Question

Level 2

361 points

Difference Between Public and Private Ports?

In setting up port forwarding on an AirPort Extreme n, I've discovered terminology I'm not familiar with. There are both "Public" and "Private" TCP & UDP port fields.

I understand the difference between TCP & UDP, but I don't get the Public vs. Private thing. Is one incoming and the other outgoing? If so, then which is which? If not, then what are they?

Additionally, when an app Just says, "UDP 407 & TCP 407, 1417-1420" like Timbuktu does, then what most likley do they mean? Do they mean Public, Private, or both?

Many thanks!

Mac Pro, Mac OS X (10.5.1), 2 X 3.0 GHz / 4 GB RAM / Vista Ultimate (Bootcamp & VMware Fusion)

Posted on Jan 30, 2008 7:44 AM

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Answer 1

Duane

Level 10

124,303 points

Jan 30, 2008 8:26 AM in response to James Reffner

The "public" port is the port which appears on the Internet side. When remotely connecting you will use that port.

The "private" port is the port used to connect to the device on your LAN.

If you wanted to host a web page your computer would default to using port 80. However some ISPs block port 80. So you would use a different public port to bypass the ISP's block. For instance you could configure port forwarding to use a public port of 8080 but a private port of 80. Then from the Internet you would use http://yourPublicIPAddress:8080 to access your web server.

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Answer 2

James Reffner Author

Level 2

361 points

Jan 30, 2008 9:18 AM in response to Duane

Duane,

Thanks for the answer! If I may ask another then?

If I have an app that's only accessible from within my LAN (iTunes & iPhoto to allow sharing), then I'd only need to open the private ports, correct?

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Answer 3

Duane

Level 10

124,303 points

Jan 30, 2008 9:21 AM in response to James Reffner

There is still some misunderstanding...

If your service is using port 548 for example, when you configure port forwarding through the AirPort Extreme base station (AEBS) you would use:

Public port: 548
IP address: IP address of device providing the service
Private port: 548

If your ISP is blocking port 548 you might decide to use port 1548. In that case you would use:

Public port: 1548
IP address: IP address of device providing the service
Private port: 548

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Answer 4

James Reffner Author

Level 2

361 points

Jan 30, 2008 10:09 AM in response to Duane

I got you. Thank you very much for your help!

Jim

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