On Tue, 16 May 2000, Ed Carp wrote:
> Dan Kegel (dank@alumni.caltech.edu) writes:
>
> > Ed Carp wrote:
> > >
> > > Dan Kegel (dank@alumni.caltech.edu) writes:
> > >
> > > > In http://www.kegel.com/mediaone.html I relate my problems when
> > > > I was assigned an address ending in .0 or .255. Even though this
> > > > was a valid address, some routers out on the internet blocked
> > > > access, assuming packets from my address were forged as part
> > > > of a smurf attack.
> > >
> > > No, sir, those addresses are not valid addresses, since most if not all hosts
> > > may respond to them. Witness what happens when you ping an address ending in
> > > either .0 or .255 - they are usually interpreted as broadcast addresses, NOT
> > > to be assigned to hosts.
> >
> > In class B network, is it truly illegal to hand out addresses ending in
> > .0 or .255? Disallowing this makes you waste almost 1% of your address space.
> >
> > Not that I think those addresses should be used... it's not practical,
> > given that they look too much like class C network or broadcast addresses.
>
> I'm not sure about a class B network - but every book I've ever read about
> network administration says to *not* give out those addresses. I found out
> myself the hard way on a Sun network, when I tried to ping 192.168.1.255, and
> *everyone* on the network responded to the ping.
That is correct, you were in a classful /24. You do not give
out the network address or the broadcast address to a host, and in
*most* cases the *last* hop router should intercept these packets.
However, there are other network setups that *do not*
distribute like this. For example when routing a /27 + /28 to a
device that then gives them out with 255.255.255.255 netmasks.
In *ANY* case, this behavior is wrong, as a router in the
middle has *NO* way to determine what network and broadcast addresses
are, it should not block this traffic. *IF* it is approp the last hop
router should take care of this.
(NOTE: This is not to say an admin cannot write filters for specific
addresses on whatever router he wants to, only that classful address
blocking *not* by administrative fiat is wrong)
---
As folks might have suspected, not much survives except roaches,
and they don't carry large enough packets fast enough...
--About the Internet and nuclear war.
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This archive was generated by hypermail 2b29 : Tue May 23 2000 - 21:00:11 EST