I just got an Ethernet shield for the Arduino and was able to get the simple "Web Server" example working from the IDE when I was at home by using ip[] = (192, 182, 1, 177) in the code.
However, as soon as I changed locations, I can't find an IP that will let me access the Arduino anymore. Is there a way to identify what IP address will work with the network you are on?
Do some research on setting up a home web server (other than the actual webserver bit which you have working) its exactly the same process. There isn't one size fits all solution, which is why no-ones done one.
You can ignore all the bits about actually setting up the webserver since the arduino has already got you to that stage. You need to know various pieces of information before you start. What kind of internet connection you have, whether your ISP uses static IP addresses or dynamic, if you have a router, what the credentials are and how you access it. In most (?) cases if you have a modern router system for your internet, you will access the router from a webpage ( something like http://192.168.0.254 but it varies ) You then log in with a username and password (varies by isp and router ) and then when you're in somewhare in the menus is usuallly a setting for port forwarding (or sometimes a default server setting). with port forwarding you need to forward port 80 to the IP address of the arduino. With a default server setting it need to point at the Arduino's IP address. You may also need to turn off some firewall or security settings or change some DMZ settings so it allows the outside world to come in.
Once thats done you may need to set the arduino so that it knows about the Internal IP address of the router (it just worked on mine but its not forced to). Once you're to that stage you should be able to access the arduino from outside by pointing it at the external IP address of your router (if you go to http://whatismyip.com/ on a browser on the inside it will tell you what this is ). Some ISPs put their own firewalling on home connections, if this is the case it will need to be disabled usualy by logging onto a control page somewhere. If you're unlucky your ISP may have a policy of not allowing incoming connections at all in which case you're stuffed until you change ISP.
If you have a static IP address thats all you need, if like most people you have a dynamic IP address, you need to sign up to a dyamic DNS service and have something on the inside updating it. I use http://www.dyndns.com for this. If you're lucky your router will have dynamic DNS software built into it and it can be set up there. If not you're going to need a machine other than the arduino doing this for you (your PC).
Like I said, its not for the faint hearted, I'm a networking professional and it took me a while to fathom out. (Notice how many people are jumping in with suggestions ? ). Its specialised and not really arduino related.