how do I remove DNS name

I have introduced a new more syntactically correct dns name for my wireless router as the old syntactically incorrect one was leading to emails being rejected by the smtp server at my work for ehlo reasons.

However my computer still identifies itself with the old dns name. I am guessing this is because this is still stored first in the network > advanced > dns search domain, although this is a guess. It is greyed out and won't allow me to delete it. Do you know how I can delete it?

My new router name is listed second and can be deleted but this is the one that works.

Thanks.

Try creating a new "Location" You'll find locations in the network system preference.