Routing to internet addresses ending with 255

[Nikos Vassiliadis]

On Friday 16 May 2008 12:32:35 Manolis Kiagias wrote:
> I had this weird problem today, and I would like to know what caused it:
>
> I have two home servers, on different locations, on two ADSL lines using
> dynamic DNS. One is running Debian, the other FreeBSD 7.0-RELEASE.
>
> I usually ssh from one to the other. Today, the debian server had a
> public (internet) IP ending in 255. The FreeBSD 7.0 system refused to
> communicate with it. Another 6.3 system had no problem. The 6.3 and 7.0
> system have identical adsl routers.
>
> Trying a traceroute from 7.0, it would seem the debian system was one
> hop away, which is of course incorrect.
>
> I understand that x.x.x.255 is ethernet's broadcast address. 

No, it's not. Since these days IP is classless, a network
(and thus its broadcast address) is completely local information,
not known to remote hosts. What might look to an external observer
as a /24 network, may be something else. For example 213.0.0.255/24
may be the broadcast address for net 213.0.0.0/24, but it's not the
broadcast address for net 213.0.0.0/23, which would be 213.0.1.255.

Also, regadless of being the broadcast address or not, to the external
observer that address is just an IP address. The router of the network
will handle specially(will broadcast) the packet if it's destined for the
broadcast address.

> However 6.3 
> had no problem connecting to it, while 7.0 would not. Has something
> changed in FreeBSD, is this the intended behaviour or a bug?

This looks like a bug. Can you post more info about it?

> Furthermore, is it valid for my ISP to assign me an address ending in
> 255?

Yes, assuming that you speak of a PPP connection. There is no
network concept in PPP. The two peer addresses are totally
unrelated. For example, a PPP interface configured with
10.0.0.1 --> 172.16.255.255 is perfectly valid configuration.

HTH, Nikos