What is used to determine if the host is local or remote?

Answer 1

It could be said that the most "local" host is the machine you are typing on, but that realization is not always implicit in the design of the protocol you are using. Also, computers on the same physical network can be considered local, as can be considered computers within the same address space - it really depends on your definition of "local" and "remote".

If you're thinking about "geographical locality", IP address groups are assigned to countries, and then sometimes subdivided into states or cities, where applicable and possible, and there exist services on the World Wide Web that access this publicly available information in order to provide at least a rough approximation of the host's geographical localization.

However, the most common idea thought about when speaking of "local" and "remote" hosts, is the idea of logical network placing - ie. a computer will be considered "local" if it belongs to the same address space (in this case, the IP/mask calculation will be performed, and will include any specific routing information, to arrive at a conclusion).

Also, any computer you are connecting to can be considered the "remote host" - even your own! That is simply because the "other end" of a connection can be called "remote" just because it's on the other side of said connection (in this case, just the connection itself will differentiate the two sides of it). As I've pointed out above, it really depends on your definition of "local" and "remote".

It is also worth keeping in mind that from the perspective of the computer you're trying to contact, you are the remote host!