computer to "smart device" - subnet question

[Lai Zit Seng]

On Sat, 28 May 2005, Jim Cornette wrote:

> Setting the netmask for the computer to 255.255.0.0 allows communication 
> between a computer with addresses in the 192.168.127.x and with devices in 
> the 192.168.2xx.x range. The problem I am experiencing is intermittent 
> dropout of "smart devices" that also have settings for masking and are set to 
> 192.168.127.xx with a mask of 255.255.255.0. The computer ethernet address is 
> set to 192.168.217.14.

If the netmask is wrong on those smart devices, they will need to route 
through their default gateway to return traffic to you.

That said, I've seen (and it is possible for these smart devices you're 
referring to) to cache the IP/ARP mapping seen from the first packets that 
your computer sends to the smart devices. That would explain why this 
appears to work. But ARP cache entries will expire. When they are gone, 
the smart devices will try to send the packet according to normal IP 
routing rules: i.e. if not in the same subnet, must route through default 
gateway. If you do not have a default gateway for them, the packet is 
lost.

> Anyway, could the intermittent dropout with communication be caused by the 
> masking on the smart device not renging with the IP of the computer or would 
> the broader setting for the computer be enough to not cause a loss of 
> connectivity.

You should really try to get the netmask set correctly. You cannot depend 
on a "netmask trick" done on just one end to make it work reliably for 
both ends of a communication.

Regards,

.lzs
--
http://zitseng.com/