ssh terminal window closes immediately when opened
Here's what happened. On the machines I was trying to reach (10.x.y.182 and 10.x.y.194), in the time when I was not using them they had their hard disks replaced and the OS reinstalled. This gave them new SSH keys, invalidating the ones I had in my ~/.ssh/known_hosts file. Unfortunately, Terminal.app did not surface this problem to me.

By deleting the entries for those machines in ~/.ssh/known_hosts, I forced the system to pull down the new keys and ask me if I wanted to use them. With this done, connections worked perfectly as usual. Oddly, however, even before I changed ~/.ssh/known_hosts, connections using the DNS name of the machine (svr10 and svr20) worked. Weird.

Thanks, everyone, who looked at this problem.