Dave Siegel writes:
Ah. here is the rub. When you ISP buddies come back, you should ask them to return the origianal /22 for a /20. That way, the total size of the routing system stays the same!
And every time they need to expand, even using the same provider, they have to renumber their whole network?
I sincerely hope that was sarcasm I detected in your post...
Renumbering isn't necessarily required if the blocks are given out such that there is room for growth should the smaller IP come back for more addresses. For example when I allocate individual class C addresses, I don't give them out consecutively. Initially I leave at least three unused addresses after the one I allocate. If someone comes back and needs more, their allocation grows to fill up the holes. I try to judge who will grow (e.g., a town network) and who won't (a BBS) and leave my options open as long as possible.