Each machine on the internet has an address, known as an IP address. For servers, this is usually static and never changes. We use this to control access to our servers; hence we need to know what it is for your servers when we set you up a webservice account.
The easiest way to find it out is to access www.myipaddress.com on your server using a web browser. It is normally a 4 part number, of the form 188.8.131.52
If you are a developer working from home or a small office, it is possible that your internet service provider has given you a dynamic IP address (changes occasionally) rather than a static one (fixed). If so, let us know and we can relax the constraints on your account to cope.