When applications operate in the global MBone, it is clear that not all groups should have global scope. This is especially the case for performance reasons with flood and prune multicast routing protocols, but it also the case with other routing protocols for application security reasons and because multicast addresses are a scarce resource. Being able to constrain the scope of a session allows the same multicast address to be in use at more than one place so long as the scope of the sessions does not overlap.
Multicast scoping can currently be performed in two ways which are known as TTL scoping and administrative scoping. Currently TTL scoping is most widely used, with only a very few sites making use of administrative scoping.