What effects does putting an internet router in bridge mode have?

Question

I had a computer connected to the internet through a router.

When I switched the router to bridge mode, the computer no longer could connect to the internet.

It gave an error about a self-assigned IP address.

The computer was running MacOS.

After spending much time troubleshooting, it was certain the computers settings were good and DHCP was enabled.

I switched the router back to un-bridged mode and it worked.

I was wondering how switching a modem to bridge mode could possible cause an issue?

If a modem is set to bridge mode then I guess it would be assigning the computer it’s connected to an external IP address instead of an private IP.

Is this right and could it mess up the computer so it couldn’t reach the internet?

Also, by router I’m talking about the ones that ISPs give their residential clients.

So it combines switch, router, modem and firewall in one. Is there a better name for these, to avoid confusion with a real router?

Answer 1

A router works connecting two or more different IP subnets (Level 3), and a bridge works connecting two LANs (Level 2).

In the following image the router connects the network 10.0.0.0 subnet to the 54.239.25.0 subnet.

The MacOs has as Default Gateway the IP address 10.0.0.1, this corresponds to the IP address on the interface of the router on that LAN.

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As the bridge works at level 2 it doesn't understand IP, so if there are different subnets in its interfaces the devices will be unable to communicate among them.

In the following image, the MacOs device is unable to find the Default Gateway and is in a different IP network of the destination network (10.0.0.0 => 54.239.25.0)

If you change your router to bridge then you will need to change the MacOs device to be on the same subnet of the external interface.