Based on IP path information, by analyzing a general model of AS border, the concept of AS border sequence is introduced, and a series of AS border judging rules that discover the AS border division law hidden behind IP path information are proposed. Therefore, a method on judging AS borders named JBR (judging border by rules) is put forward. The experiment results show that JBR’s judging time is shorter than the method named JBA (judging border by alias), which is based on alias resolution, and it has advantage on judging both border addresses and border links.