I have a fairly old router that doesn’t support gigabit. I also have a network switch that does support gigabit. If I connect two devices directly to the switch, then connect the switch up to the router, will the connection between the two devices support gigabit? If I’m understanding correctly the router would just act as DHCP server and give the two devices a local IP address, but the actual connection between them wouldn’t go through the router at all.
yes, that is generally how it works. There are some edge cases and possible misconfigurations that would slow things down, but in general data between the devices on the switch will be gigabit and to/from the router (and thus the internet) will be limited to 100mbit.
Assuming of course that the uplink to the internet is 100M. And that the router with firewalls, tunnels, NAT and all can actually push whole 100M trough. That’s a pretty safe assumption with 100M, but I’ve seen devices which technically hve a gigabit ethernet connection but with real world traffic the routers CPU is a bottleneck and it’ll limit speeds well below that.