And yes, there is an incentive to return space. For some its the principle of the thing, a cooperative internet is a growing internet. For others it may be financial, with the expense of renewing lease delegations. (See the naipr/arin lists for more details).
In conversation with some NANOG participants last week I was asked more than once for clarification or further details about 192/8, continuing from Bill's remarks Friday. It does seem that 192/8 isn't the concern it was eighteen months ago. One reason is that some carriers now refuse to route legacy 192/8 delegations for new customers, requiring instead that they renumber into provider space. Folks holding delegations but not yet trying to route them were never an enormous number of prefixes but looked for awhile like they might be a significant contributor to routing table growth; back-of-the-envelope calculations a little while ago suggest this potential doesn't seem to have materialized (it's hard to be sure because routing table announcements don't have origin dates, but there's some reason to believe new announcements are simply better aggregated than older ones, even in 192/8). An informal survey: how many NANOG participants have asked customers to renumber out of 192/8? And for how long have you been doing this? IMHO the remaining major obstacle to a concerted effort at reclamation in 192/8 is the database maintenance problem. Folks with no incentive to keep their whois entries current have not been doing so-- since the typical pre-CIDR "Class C" delegate was not an ISP and has never come back for more space, the registries have limited leverage over them, with a few notable exceptions such as DDN-NIC. Accordingly, "whois" contact information simply doesn't tell you who's using a block. I haven't risked a blind survey again since the first one, but I have no reason to believe this has changed-- any takers? I hope and expect that ARIN may be able to throw some organized effort at this problem, if the membership feels that cleaning up the database is important. Suzanne Woolf woolf@isi.edu (ISI pays me to run routers'n'DNS'n'things, my opinions are mine)