What's this RFC1918 stuff you keep talking about?

RFC 1918 defines a set of 3 IP ranges that are not routed across the Internet and can only be used on local networks. You will sometimes see these used on ISP networks, where the devices can only be accessed from within the ISP's network, not from the rest of the Internet.

There are 3 IP ranges defined in RFC 1918:

10.0.0.0/8 (10.0.0.0-10.255.255.255)
172.16.0.0/12 (172.16.0.0-172.31.255.255)
192.168.0.0/16 (192.168.0.0-192.168.255.255)

If you ISP or provider allocates one of these IP ranges as your WAN IP then your device can only be reached from within your ISP at best. They are very commonly seen where the provider is part of an academic environment (school, college, hall of residence etc), is part of an apartment complex or a mobile phone network. They are also normally seen where the ISP connected device is an ADSL device and you are connecting a router to that device (discussed briefly here).

The problem is that if you visit any of the web pages that claim to tell you your IP address they will tell you the IP address your ISP is NATing you behind. The only way to know is to look at the management pages of your ISP connected device (or if you are connecting with a USB connected modem looking at the IP assigned to that interface - using ipconfig on Windows and ifconfig on Linux and BSD).

For over a decade those IP ranges (and the link-local subnet) have been routed to AS112, which is a black hole designed to minimise the impact of people trying to reach those IP addresses across the Internet.

There are also a few other special case IP ranges that you may also see, covered in RFC 3330 and RFC 3927:

0.0.0.0/8 (0.0.0.0-0.255.255.255)
127.0.0.0/8 (127.0.0.0-127.255.255.255)
169.254.0.0/16 (169.254.0.0-169.254.255.255)
192.0.2.0/24 (192.0.2.0-192.0.2.255)

These IP addresses also cannot be routed across the Internet, some (127.0.0.0/8) cannot be routed beyond the local host and the others beyond the local network.