Question Ethernet has a self assigned IP address/DHCP ?

Sep 18, 2021
House has been wired with Cat5e, was meant to use Cat6. Now NBN is connected we are trying to sought out issue with house network,
I am trying to connect my iMac El Captain desktop via ethernet cable. I can connect through wifi. (Also trying to connect Sony Smart Tv.)
I have connected my iMac to NBN and modem directly with ethernet cable, upstairs in comms room. When I take desktop downstairs and try and connect via ethernet cables I can not get a connection.

I have tried renew DHCP lease for ethernet connection and I have the error " Ethernet has a self assigned IP address and will not be able to connect to the internet"
In the ares that I am trying to connect there is a RJ12 coming out of the wall with the the other end being RJ45 going into a switch. The ethernet cable is coming out of the switch and into my computer.

Lights are "on" on the switch for the cable going into my computer and rj45.
NBN - Modem - ethernet cable into wall to downstairs - rj12 - rj45 - switch - ethernet cable - imac
Spent most of the days trouble shooting with service provider.

Cablers said we needed a second modem where we have placed the switch, and that they would return a swap the rj12 jack over to ethernet jack.
Went to purchase modem and was told by sales that we need a switch instead.
So I take it the DHCP is not making it through the ethernet cable? Any manual work around? Is it a problem that cables are cat5e not 6?
rj12 is telephone lines not data. Cables that have rj12-rj45 are mostly used for a DSL modem connection not to connect to ethernet.

Your best bet is to pull the rj12 jacks and replace them with rj45. Getting the proper wire pairs and colors on the correct pins is key to making ethernet work. Telephone doesn' t care as much.

If the cable is really cat5e it will work fine cat6 provides no benifit.

When you get the correct jacks you should see a light on your switch when it is plugged into the modem.

Note if the device you call a modem is really just a modem and not a modem/router you can only plug 1 device into it so a switch will not be of much use.
Sep 18, 2021
Thank you for your response.

Yes old tech had installed a plate with 5 ethernet/1 telephone.

The modem upstairs is router/modem. It has 4 ethernet ports. One ethernet comes downstairs into the back of the rj12, and one goes direct into another room downstairs.

New techs did say to plug rj12/rj45 from plate into a second modem, so that make sense. New tech where also meant to come back and swap the plate rj12 for a rj45.

Is there anything else we can do? Can we change the rj12 on the plate? As still waiting to hear back from tech?
As long as the wire really is ethernet cable it is very easy to swap the wires. You just remove the wires from the rj12 likely trim off a small amounts and punch them into the rj45 keystone matching the wires to color pattern on the side. Many home improvement stores sell rj45 keystones and many of these do not need special tools to install.