Integration of space resources (e.g., satellites) in different orbits with the terrestrial/ground network (e.g., internet) is the new evolving era of information technology. This kind of information network can expand the ground services such as internet to the whole world with the help of satellites' broader coverage. On the other hand, it can also provide realtime access to the satellite's useful data (e.g. images of earth), for nearly everyone on the globe. For these reasons, there has been enormous amount of research on this topic, and different network architectures are proposed for the integration of space and ground networks for different applications. However, most of them are either general abstract architecture or for the single dedicated application. Also for such a network, it is desirable that it should be based on the TCP/IP (Transmission control protocol/internet protocol) stack for interoperability with terrestrial networks. However, the biggest challenge for this is the dynamic network topologies due to orbital dynamics of space nodes. In this paper, we have addressed both of these issues by proposing a Space-Air-Ground Information Network (SAGIN) Architecture, which can provide almost all of the services of space and ground networks for all the users around the world according to varying needs of users. Our proposed SAGIN architecture is IP-based to provide interoperability and compatibility with the already deployed ground network. We have also addressed the challenge of dynamic nature of the network topology by designing a simple mechanism which automatically maintains a relatively static IPs of nodes in space (satellites), despite their rapid physical movement in orbits. This SAGIN architecture could be among optimized candidate architecture for future to have for many countries (either individually or by the cooperation of different regional countries). Because, it can provide all the services to nearly everyone in the world, in contrast to traditional individual satellites providing limited services to a limited number of users.