On Sat, 14 Mar 1998, Blake Willis wrote:

It _does_ break things if not done with very careful attention. Most of the people using it have no clue about how it can break things and are bewildered when it is explained to them.

For example? A router with one ATM interface going to the world with a high MTU with an ethernet on the other side. Say you use private IP space for links on that router. Say someone on the Internet filters traffic from private netblocks; lots of people do. There _can_ be machines that are completely unable to transfer data (eg. download a web page) from another because you just broken path MTU discovery. This is not a made up situation, this is a real example that I have had to deal with of how using private IP space for network interfaces used for public traffic does break things in some situations.

Also protocols like CUSeeMe that embed IP addresses in the protocol will break for any leaf nodes that have a private netblock address.