When running a router as a switch can the WAN port work as 5th LAN port?

[TnF]

Usually when i have an extra old or used router i set it up as a wired and wireless access point around a house;
I generally disable the DHCP server, set a known fixed IP so i can access it later, and use one of the LAN ports on it so it can act as a switch.

But i have this question in my mind; what happens to the WAN port? Can it function as a 5th port? It depends on the router (on some you can put it in switch mode). What happens if there is no dedicated mode available? Is there a solution if you need 5 LAN ports in a pinch and you have no other hardware on hand?

Also does the internal switching hardware affect the bandwidth? Example on my old RT-AC68U, while it has all 1gbps ports, in practice each of the 4 LAN ports maxes around 250mbps and only the WAN port can do a full gbps (or close to it).
Meaning if you are using it as a switch and the WAN port can act as LAN, you are better connecting that to the upstream network so you have proper distribution of bandwidth?

So many questions ;p

 

[Tech9]

What happens if there is no dedicated mode available?

In this case the WAN port remains as WAN port and can't be used for LAN switch purposes.

Example on my old RT-AC68U, while it has all 1gbps ports, in practice each of the 4 LAN ports maxes around 250mbps

All ports are the same up to Gigabit on this model. The uplink port bandwidth (any port can be used) is obviously shared between all other ports when they are used at the same time. Asus router in AP Mode turns the WAN port into LAN port.

 

[ColinTaylor]

As you have posted this in the non-specific "Routers" forum the answer is: It depends on the specific device.

Using the RT-AC68U as an example, yes you can do it. The best way would be to configure it to AP Mode and turn off the radios. All ports are capable of gigabit throughput. If you weren't getting that then I'd suggest your testing method was flawed.