An Internet Service Provider, or ISP, number is a unique string of numbers separated by a trio of dots to identify a computer's connection to the Internet. They are assigned to households or businesses by the service provider during installation.
If using a router, two ISP numbers are assigned, external and internal. The latter is assigned by the router itself. Almost all residential ISP numbers are static, while businesses and large network facilities are dynamic. ISP numbers can be changed for a number of reasons. The most common are changes in service or speed, technical difficulties or switching service providers completely.