Find your Public IP Address from a Router

Find your External / Public IP Address

Introduction to Port Forwarding.
Turn on the server on your Mac.
Open your Mac's firewall for that server's port.
Give your Mac an appropriate static IP Address on your Router's sub-network.
Set up your Router to pass external access requests to the Mac running the server (Port Forwarding.)
Determine your external IP Address for use out on the internet.
Connecting to your home File Server from the Internet.

Find your External / Public IP Address

You – or anyone else – connecting to your Mac from the internet will connect to your public or external IP address, the one assigned by your ISP to your Router on its WAN port. For many services, you'll need to tell the person using the service what that IP address is. (Some services, like file sharing applications Gnutella and BitTorrent, learn your IP Address when you contact a peer server.)

You can find your public (aka external) IP Address in a number of ways. Maybe the easiest is just to connect to a website such as WhatIsMyIP.com and see what IP address you're using.

You can usually determine your external IP address from your Router's admin utility. For example, it shows up in the the Status screen of the LinkSys admin pages:

Or, it shows up in the Airport Admin Utility's Summary screen as the "Public (WAN) IP Address":

Alternatively, you could just arrange to send periodic eMail to anyone using your Mac remotely. Most ISP's SMTP outgoing mail servers include your current IP address in the header of your eMail. Using "Source" from Mail's or Entourage v.X's View menu will let you look at the full internet mail header, where you'll probably find a header item with the sender's public IP Address.

Keeping Track of a Changing IP Address

If you have a problem with your ISP changing your IP address too often, you might want to sign up with Dynamic DNS service. They can give you a symbolic name (e.g. yourname.dyndns.org) that it will convert into the current IP address of your system, along with the tools, such as DNSUpdate, that run on your Mac, check your current IP, and keep it up-to-date automagically (configure it to use your external IP address when checking). I have even seen some newer routers, such as the NetGear 814, that include the ability to keep DynDNS aware of your IP address automatically - if your router supports this, take advantage of it! Then you or others can just use that name to connect to your server and always get connected at your Mac's current IP address.

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