Every device that connects to the Internet must have its own, unique IP number. These are assigned centrally by a designated authority for each country.
Please note that the IP numbers used throughout this document are FICTITIOUS! If you are connecting to the Internet, YOU MUST use an IP number from your own assigned range. You MUST NOT pick IP numbers "out of the air" and use these whilst connecting to another LAN (let alone the Internet) as this will cause total havoc.
There are IP numbers that are specifically dedicated to LANs that do not connect to the Internet. One of these sequences is 10.0.0.0 to 10.255.255.255. This is the sequence I have used as examples in this document.
For most users, who are connecting a single machine (or a small LAN) to an Internet service provider via PPP, obtaining an IP number (or more accurately, a network number) will not be necessary.
If you wish to connect a small LAN to the Internet, many Internet Service Providers (ISPs) can provide you with a dedicated subnet (a specific sequence of IP numbers) from their existing IP address space.
This will almost certainly cost you (significantly) more than a simple dial up connection using a dynamic IP number assigned to you automatically (and temporarily) when you connect to your ISP.
For users, who are connecting a single PC to the Internet via an Internet service provider, most providers use dynamic IP assignment.
That is, as part of the connection process, the PPP service you contact will tell your machine what its IP number is for the current session. You are not given the same IP number each time you connect.
This has implications for server type applications on your Linux machine such as sendmail, ftpd, httpd and so forth. The limitations of service due to dynamic IP number assignment (and ways to work around these, if possible) are discussed later in the document.