The default VLAN on all switches is VLAN 1.
By default, all ports in a Cisco switch belong to VLAN 1. With all ports in the same VLAN (in this case VLAN 1), all ports can communicate with each other without going through a router.
However, if you change the VLAN assignment for a switch port to another VLAN, that switch port will not be able to communicate with the rest of the devices on other ports. In this case, you will need a router to route packet from one network to another network.