True enough. But it would cut out most of the the problems with really small customers (such as the /30's which were mentioned ;-) Sure there are some cases that are hard or impossible to account for, but it would be helpful for ISP's to take a stance that "here's your address space. Unless you tell us otherwise, we assume its not subnetted, and we will block your broadcast addresse. If you subnet or otherwise change your broadcast address, you will need to notify us" I guess what I'm saying is that for many, the default is to do nothing. The default should be different where possible. --Dean

That works fine as long as you either manage your customers' equipment or your customers don't subnet blocks you give them. However, in real-world experience, neither of those apply, especially to a larger ISP/NSP (UUNet was mentioned in this thread at the beginning).

It certainly doesn't hurt to put in access-list's where you can, to reduce the problem, but that is not a scalable solution. It is an incredible management nightmare, especially if you're having to keep track of autonomous customer routing changes. Not to mention that it adds to the burden of tracking down problems (imagine a DHCP server which assignes what used to be a broadcast address, but is no longer because the subnets were combined, and everytime a machine gets that address, it can't get outside the network because the administrator hasn't updated the 5,000-line access-list).

Pete.

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