> Add a one-byte option into the header, give it an unassigned value (e.g. 6) and assume that any box that does not understand it will pass it through unchanged

> Add a one-byte option into the header, give it an unassigned value (e.g. 6) and assume that any box that does not understand it will pass it through unchanged. As many have found out, unfortunately "any box that does not understand it will pass it through unchanged" is frequently false. What often happens is that a box which does not understand it silently drops the whole packet, under the mistaken impression that it's an attempt to invade the network. > Use it as the most significant byte of the address, and obsolete all routing equipment that doesn't understand it from the backbone You'd have to not only obsolete all routing equipment, but also all end hosts. Consider what happens when host A, which does not understand the option, receives a packet from host B, which has an address which requires the option; the reply packets from host A will never reach host B, since host A doesn't know it has to add the option. These "just add another byte" solutions are often proposed in these discussions, and look deceptively simple at a first glance, but fall apart once you start looking at it in detail.