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

James R. Cutler james.cutler at consultant.com
Tue Jan 8 21:52:27 UTC 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
james.cutler at consultant.com






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