The need for a modem between an ONT (Optical Network Terminal) and a router depends on the type of internet service you have. In traditional fiber-optic setups, the ONT is the device that converts the optical signal from the fiber optic cable into an electrical signal that can be used by your router. However, in some configurations, a separate modem may not be necessary. 

Here are two common scenarios:

1. ONT directly connected to the router:

• In many fiber-to-the-home (FTTH) installations, the ONT may connect directly to your router. In this case, the ONT performs the function of a modem by converting the optical signal into an Ethernet signal that can be used by your router.

2. ONT connected to a modem and then to the router:

• Some internet service providers deploy a separate modem between the ONT and the router. In such cases, the modem is responsible for converting the optical signal into an electrical signal that the router can understand.

If you are unsure about your specific setup, you can check with your internet service provider or refer to the documentation that came with your equipment. In many modern FTTH installations, especially those using Ethernet-based connections, a separate modem is often not required.

It's worth noting that in fiber-optic setups, the terminology can sometimes be used differently by different providers. Some may refer to the ONT as a modem, while others make a clear distinction between the two devices. Always refer to the documentation provided by your service provider or contact their support for accurate information about your specific installation.