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Re: Using x.x.x.0 and x.x.x.255 host addresses in supernets.

  • From: James R. Cutler
  • Date: Tue Jan 08 16:54:25 2008

I am astounded at seeing this discussion. I have not seen this much disavowing of CIDR addressing since 2003 or before.

At least these arguments against .0 and .255 IPv4 addresses are based on perceived cost of operations, not ignorance of effective network number vs effective host number. Now, if we can get Microsoft to really support TCP/IP, we can make much progress. Of course, ubiquitous deployment of IPv6 will fix all that.

Especially on proxied enterprise networks, use all the addresses available base on the effective network address having host number of 0 and the broadcast address being an effective host address of all ones. We have had much success with this approach for some large customer networks. Also, if your router OS works in a classful manner, tell the vendor to fix it. We got CIDR years and years ago.

Note, the referenced Microsoft article uses the phrase, "the client may have difficulty communicating", not will.

On Jan 8, 2008, at 4:12 PM, David Schwartz wrote:



Historically, .0 and .255 have been avoided because a lot of servers
(windows) wouldn't work using that as a host address or would flag it
as invalid if you tried to connect to it or a myriad of other
problems. Note that this was a limitation of the host, not anything to
do with the network or any of the network equipment.

This issue has not existed with any prevelance for quite some time and
almost everything of recent manufacture is quite happy to be assigned
in a supernet as well as on the .0 and .255 addresses.

So my oppinion is don't hesistate to use it until you find a real,
reproducible problem.

-Wayne

I have seen networks where traffic to these addresses was filtered in an
attempt to mitigate broadcast address amplification. Typically, end users
filter their inbound Internet traffic to their own addresses. They know they
don't use .0 or .255 addresses and they found this is a quick way to prevent
any nodes on their internal network from being used as amplifiers without
having to audit/fix their entire internal network.

As we know, the "workaround" may remain in their edge router(s) long after
it has outlived its usefulness.

A few years ago, I noticed that an ISP blocked all traffic from its
customers bound for any .0 or .255 address to prevent drones from flooding
those addresses. I doubt this is typical, but I bet it's still around in at
least a few places.

If you're seriously considering using these addresses, these are other
possible issue you need to consider.

DS



James R. Cutler
[email protected]