OS 10.10 not connecting through proxy specified by its Internet name My office proxy has always been specified with its IP address and everything worked fine. However, recently, the proxy was changed to use its Internet name, in the form xxxx-y.zzzz.com 

Since then, I have been unable to connect. I keep getting infinite authentication dialog boxes, but still doesn't connect. I've opened /etc/hosts to add its Internet name and map it to its IP address. It is still failing. 

However, if I change back to the legacy numeric iIP address, it connects fine. Problem is that the legacy IP address will soon be discontinued as it has issues.
The alphabetic xxxx-y.zzzz.com is a hostname, not an ip address, and hostnames almost always resolve to an ip address. You should try doing an nslookup in Terminal.app to confirm whether or not you've got the right information in your hosts file.

$ nslookup xxxx-y.zzzz.com 

Non-authoritative answer:
xxxx-y.zzzz.com canonical name = p9p.geo.asbs.yahoodns.net.
Name:    p9p.geo.asbs.yahoodns.net
Address: 98.139.134.174


Then in /etc/hosts confirm you've got:

98.139.134.174   xxxx-y.zzzz.com


something tells me xxxx-y.zzzz.com is not your real hostname, but you get the idea.