About IP addresses and hostnames
An IP address is a number that specifies a specific computer on a network. It looks something like this
64.130.46.127 - An example IP address. This is the address of the main Nakahara Informatics web server.
A hostname is an alphanumeric name that points to an IP address. Hostnames help make it easier for humans to remember machine addresses, and they look something like this:
www.nakahara-informatics.com - An example host name. This hostname points to IP address "64.130.46.127", which is the main Nakahara Informatics web server.
Nowadays, your Mac may actually have multiple IP addresses, and multiple hostnames, at the same time. Some IP addresses only work on a local network, and are called "private" IP addresses. IP addresses that work on the entire Internet are called "public" IP addresses. When you are connected to your local network, you can access your Mac using a private IP address. However, to access it over the Internet, you must use a public IP address.
Your "effective" IP address
There is not yet a standard term for this last one, but for the purposes of iGet Mobile, we've dubbed it the "effective" IP address. This is the address you actually use to access a computer via the Internet, even though that IP address may actually point to a router, and not the Mac itself.
For example, let's suppose you have two Macs, and a single DSL connection from the phone company. The DSL service automatically supplies you with a single public IP address, but you set up your network to share the connection between the two machines, via your Airport base station.
In such a scenario, the Airport base station actually has the public IP address assigned to you by your ISP. The Airport, in turn, is configured to forward port 5000 to your iMac, and port 6000 to your Mac Pro. Each computer is running iGet Mobile, set to use these port numbers respectively.
Well, although neither of your Macs really has the public IP address, this is the IP address you use when connecting from outside. You are actually connecting to your base station, which is forwarding the connection to the appropriate Mac.
In these situations, we refer to the public IP address as the "effective IP address" of your Macs, because that is effectively how it works. You enter this address into your iPhone, add the appropriate port number, and—voila!— you are connected to the appropriate Mac.