The Classic Era realms were conntected by timezone & server type, commonly referred to as ‘clusters’. The realms within a given cluster are effectively merged: players can fully trade/guild/raid/etc with others within their own cluster. RP realms were exempted from these connections. Below is a portion of a Blue post detailing realm connections for the 5 North American clusters and the 1 Oceanic [server type/timezone added]:
Additionally there are free transfers available to the pve east realm on Mankrik and pvp west realm of Whitemane.