.255 addresses still not usable after all these years?

bmanning at vacation.karoshi.com bmanning at vacation.karoshi.com
Sat Jun 14 00:07:15 CDT 2008


On Fri, Jun 13, 2008 at 03:08:47PM -0400, David Hubbard wrote:
> I remember back in the day of old hardware and operating
> systems we'd intentionally avoid using .255 IP addresses
> for anything even when the netmask on our side would have
> made it fine, so I just thought I'd try it out for kicks
> today.  From two of four ISP's it worked fine, from Verizon
> FIOS and Road Runner commercial, it didn't.  So I guess
> that old problem still lingers?
> 
> David
> 

	well... .0 and .255 are still special in -some- contexts.
	they still form the all-zeros and all-ones broadcast addresses
	for the defined block... so:

	192.168.16.0/23

	192.168.16.0/32 is unusable
	192.168.16.255/32 is useable
	192.168.17.0/32 is useable
	192.168.17.255/32 is unuseable.


	crapy CPE, vendor instruction, poor software all contribute 
	to VLSM being poorly understood and these "gotchas" still 
	around - years - later.

	my recommendation... place your caching nameservers and webservers on
	these addresses... if you want to force the issue. :)

--bill




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