Help connecting directly to modem using comcast

Hello all, I work in a number of different houses all with different computer settings, so I bring my computer everywhere. It is an iMac. I'm having trouble connecting via ethernet into the cable modem at one place. I can connect my xbox directly if I manually enter the IP and DNS info. I know that the service is comcast and the modem is an Arris. It wont detect an IP automatically, I have tried to enter the info manually and it says ethernet is connected with the ip blah blah blah....but it wont connect to the internet or anything. Anyone have any ideas??

Thanks

 

didn't work