additionally, how is the ISP to account to ARIN for this block should they go back for more space?
They show ARIN a copy of the TRO. Really.
there is a widely accepted understanding of how this is all supposed to work, and if the ex-NAC customer succeeds in gaining this TRO, and it becomes a pattern across the industry, then everybody's connectivity, router tables, and support budget will likely suffer.
Unfortunately, courts are generally not impressed by the "if everybody did it" argument. In each case, they'll look at the actual incrementlay harm the TRO will do in that particular case. I do agree, however, that it's very important that if these cases do come to court, the ISPs win as many of them as possible. Each loss makes the next loss easier. The reason I'm pointing out which strategies are unlikely to work is not because I hope they won't work but because I want him to make sure to emphasize the strongest possible arguments. IMO, these are: 1) The emergency is of the customer's own creation. He had plenty of time to renumber and didn't. 2) There is no significant harm in renumbering immediately. Techniques such as 1-to-1 NAT exist for exactly this purpose. 3) DNS creates a portable namespace, so there's no legitimate portability issue here. This is not like bringing your phone number with you when you change providers but like bringing your phone *line* with you when you move across the country. I would not try to argue that it harms you significantly to allow them to continue using the IP space. I just don't see any honest way to show that there's real harm. Perhaps in your specific situation there's such a way. But defending abstract principles about router table sizes isn't likely to work. The court will weigh the harm of the one advertisement. Don't forget, though, the customer can't get the TRO, regardless of the balance of the hardships, unless they're likely to be entitled to it. So look into the details of how the contract got terminated, and if it's because of the customer's own actions, they're not likely to be entitled to the relief the TRO seeks to get anyway. DS