Routers can get quite complicated.

At a first level, routers will cache information about what machines are on which port of the router. This enables them to not forward a packet to every port on the router (reducing network load). This is normally done using MAC Addresses.

On more expensive routers, the router can understand IGMP, or multicast and route based on multicast joins. This enables an optimization on simple broadcasts because not every machine on the network needs to see these packets, but multiple machines might want to.