As i know bridges and switchs reduce collision domain and routers only reduce broadcast domain . But can a router reduce a collision domain ??
Actually no, a router will not reduce a collision domain because it is at the boundary of the collision domain (as pointed out in comment by Ron Maupin).
There's an ambiguity in the word "network", you can use it to speak about several related but different things:
- a LAN (Local Area Network) which usually refers to all interconnected devices on a site and can span many IP networks
- an IP network: a defined range of IP address that can be subnetted and span several layer 2 networks
- a layer 2 network : a collection of devices and switches connected together, which correspond to a broadcast domain. VLANs muddy a little bit this definition, since you can have several layer 2 networks on a single switch, but the same principle apply.
When talking about routers we consider IP networks (layer 3), while collisions occur at layer 2, so the two concepts are somewhat unrelated.
If you have a single layer 2 network connected with hubs, you have a single collision domain. If you split this network in two with a router then you will reduce the collision domain BUT this imply to reconfigure all the hosts behind the router with new IP settings, so this is a totally different thing that simply breaking up the collision domain.
So to keep it simple:
- hosts connected with a hub (increasingly rare nowadays): single collision domain
- hosts connected with a switch: each link between a host and the switch port is an independent collision domain.
- routers don't connect host at layer 2, they connect several layer 2 networks at layer 3.