There might also be larger subnets that contain it, but it is impossible to define a subnet (in CDIR notation) that includes some of the address space covered by x.y.0.0/16, and some address space that is not covered.

Indeed. I wasn't aware of that property of CIDRs, though I now see it is obvious.

You also talk about merging overlapping subnets. This is not what I am trying to do. The end purpose is to produce a reports of all the overlaps which will be used by the network infrastructure people to reconfigure the routers and DNS/DHCP servers so that the subnets no longer overlap.

When the infrastructure people get their hands on your report, won't one of the things they might do be to consolidate (say) 0.0.0.0/30 & 0.0.0.4/30 into 0.0.0.0/29 thus reducing router table sizes?

Or dropping this lot:

0.40.0.0/15 
0.40.0.0/16 
0.47.0.0/16 
0.42.0.0/18 
0.44.128.0/18 
0.47.192.0/18 
0.43.0.0/19 
0.47.64.0/21 
0.47.192.0/21
[download]

Because they are all already covered by: 0.40.0.0/13? (ie."coalescing" them.)

Anyway, thanks for posting an interesting question. I guess if you don't find Re: Algorithom to find overlaping subnets (Internet IPv4) useful, someone else might :)