Networking – 100Mbps and Gigabit switch on the same LAN

Yes, the router will only allow the machines connected to it to function at 100 Mbps. So, no, they will not be able to communicate at gigabit speeds when connected only to the router.

However, since you have a gigabit switch. Connect all the machines to the switch and then run the switch uplink to the router. The port used as the uplink will connect to the router at 100 Mbps, and the rest of the ports on the switch will function at 1000 Mbps. I have this exact setup. All the machines can then communicate at gigabit speeds on the switch and your 10/100 router can still be used for internet/wireless/etc.

Gigabit or 100Mbit (or 10Mbit) will be decided on a link by link basis, and it is possible to mix the two within a network. Each router/switch should automatically negotiate the fastest link speed (although sometimes it's better to specify this manually, if possible).

So, it makes sense to group all 100Mbit hardware with your existing 100Mbit switch; likewise for the Gigabit hardware and router. It will not slow down the Gigabit section of the network.