Network monitor

Hello,
I'm trying to build a small network monitor using an old router, which I want to pass traffic through and monitor traffic between devices. Something similar to wireshark/ifddler
Issue with wireshark/fiddler (that i have read), most switches don't allow you to sniff traffic, (i think i have an old hub somewhere but i need to dig around), so i wanted to build a plug and play between 2 network devices. For example,
Internet HUB <--> router, i can just plug a man in the middle, Internet HUB <--> Sniffer <--> router.
I'm hoping I can utilise the LAN ports, i.e. LAN1 LAN2, and use LAN3 to connect my PC to read the traffic.

ps, i didn't want to use the WAN <--> LAN as i don't want to mess up the IP address assignments already setup.

Does anyone have any suggestions or pointers on how to go about this?
Thanks

If you bridge the downstream and upstream interfaces on the intermediate device, it should be easy.