From: "Steven M. Bellovin" <smb(_at_)research(_dot_)att(_dot_)com>
In message <BB2831D3689AD211B14C00104B14623B1E7569(_at_)HAZEN04>, "David A
Higginbot
ham" writes:
I agree! Why create a finite anything when an infinite possibility exists?
On another note, I have heard the argument that a unique identifier already
exists in the form of a MAC address why not make further use of it?
Would it surprise anyone to hear that all of that was considered and
discussed, ad nauseum, in the IPng directorate? That's right -- we weren't
stupid or ignorant of technological history. There were proponents for
several different schemes, including fixed-length addresses of 64 and later
128 bits, addresses where the two high-order bits denoted the multiple of 64
to be used (that was my preference), or CLNP, where addresses could be quite
variable in length (I forget the maximum).
But the first thing to remember is that there are tradeoffs. Yes, infinitely
long addresses are nice, but they're much harder to store in programs (you can
no longer use a simple fixed-size structure for any tuple that includes an
address) and (more importantly) route, since the router has to use the entire
address in making its decision. Furthermore, if it's a variable-length
address, the router has to know where the end is, in order to look at the next
field. (Even if the destination address comes first, routers have to look at
the source address because of ACLs -- though you don't want address-based
security (and you shouldn't want it), you still need anti-spoofing filters.)
I should add, btw, that there's a considerable advantage to having addresses
be a multiple of the bus width in size, since that simplifies fetching the
next field.)
Routers may use the different addresses for routing. Outbound router
may assign "route address" to keep intermediate route tables small.
It is not the same as NAT because original and real destination address
never replaced.
- Leonid Yegoshin.