Subject: [dhcwg] Question re. interpretation of option 33 destination addr

From: Richard Johnson <[email-masked]>
Date: Fri, 9 Jul 2004 18:24:46 -0700

I have a question about the interpretation of the destination address 
in option 33.  Looking at RFC2132, it's not really clear how this 
address should be interpreted.

Since we now have a "classless static route" option, it seems clear 
that option 33 was a "classfull static route" option.  That being the 
case, it seems clear that given a destination address of "10.1.1.0", 
the subnet mask which should go along with this would be the classfull 
mask of 255.0.0.0.  Fine.  But given a destination address of 
"10.1.1.0" (and an assumed mask of 255.0.0.0), is this a host route?  
It would seem so.  But is that what was intended?  Or, instead, should 
the client system AND the destination address with the classfull mask 
in order to produce a subnet route (in this case "10.0.0.0/8")?

My first thoughts would be that in order to keep as much flexibility as 
possible, the destination address should NOT be AND'ed with the 
classfull mask and if the destination address happens to have bits set 
in the "host" portion of the address, then it should assumed that this 
is a "host route".  If a "subnet route" were intended, then bits would 
not have been set in the "host" portion of the address.

The real question, however, is how is this currently handled and what 
is the expectation?

/raj

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From: Ted Lemon <[email-masked]>
Date: Fri, 9 Jul 2004 22:41:34 -0700

I think the real answer to this question is that the reason we trashed 
option 33 is precisely that there is no sensible way to answer the 
question you have asked.   Sure, you could try to impute some subnet 
mask on an obviously invalid network destination, but you really 
couldn't do it safely.   For example, is 10.0.2.0 a /24 or a /23?   Do 
we impute subnet masks on octet boundaries, or on bit boundaries?

So I think the right answer is to just use the classless static routes 
option if you want to have network destination addresses that are not 
consistent with classed routing.   I think this is why option 33 is so 
infrequently used - it just doesn't work.