Hi, I have some problem with my setup.
I have a Mega with W5100 shield, and if I try to connect it in this order
Internet -> Router -> Switch -> Arduino, it fails to get ip (DHCP failed)
but if I try this order
Internet -> Router -> Arduino it works.
I tried the Switch with a laptop and that worked instantly, so no problem with that (same cable aswell).
I found this topic, but I don’t speak spanish and I don’t want to solder blindly
Any idea? (No I can’t get a direct cable from the router)
Is this related to Blynk at all? 
Sorry you just created an account and ask an irrelevant question…
At least don’t mark it as “Issues and Errors”! Changed to “Need Help With My Project”
Is it unmanaged switch or managed one? If its managed, make sure that network configuration on that switch is 100% correct. On unmanaged switch there should not be any problems because switches works with MAC adresses. Since switch is on layer 2, it must know where to send frames when it receives one on a port. To do so, it looks at the switch table, or the MAC address table.
If there is no entry in the table, it floods the frame across all ports, When a device replies to the frame, the switch takes the source MAC address from the frame and adds it to the switch table(MAC address table). That’s why working unmanaged switch shouldn’t interfere with DHCP server.
If the problem persists, start thinking about IPv6. You could get it for free in our country if you ask your ISP for it. I believe its the same in yours. IPv6 doesn’t need any port forwarding or DHCP servers.
The switch is an unmanaged one. Seems like the link I mentioned was the problem. The chinese W5100 shield has a wrong resistor on it and it is not works with all router/swtiches.
I marked it as an error, because I tested out all the equipment I had and everyone of them passed oneway or other, so I thought it is a bug / config error with the library. (all ethernet leds were fine an lid).
Thanks for your help Guys! 