sandbox.authorize.net and test.authorize.net are both down for me from one location, but if I remote to a computer on a different connection they work. So it seems like a routing issue somewhere. traceroute keeps looping back through one IP address and eventually hits the hop limit.
We're aware that some connections are unable to reach our sandbox and have reported this to our operations center.
I'd recommend subscribing to this topic so that you'll be alerted via email if there are updates. To subscribe, click Topic Options at the top of this thread and then select Subscribe. You'll then receive an email once anyone replies to your post.