[clug] ARP & ping issue

[Hal Ashburner]

I have a new adsl modem which runs linux and came with the all the source
code on a cd.
ASUS DSL-N55U - apparently I'm loyal to asus, possibly above and beyond
what I'd recommend to others.

I have a machine attached to this by wireless. It frequently stops
responding to ping from my laptop, also attached to the router via wireless.

I can telnet into the router and from the router, ping responds fine.

arping from the router does not for the machine that appears to have
dropped of the network. There is still an entry in the arp table which, I
guess, is how ping continues to function.
>From my laptop it does not appear in the arp table and doesn't respond to
ping.

After a while it comes back, arping and ping respond from everywhere.
Clearly the machine is up if it responds to ping from the router, it's just
hidden from the rest of the local network sometimes.

The "disappearing" machine is using NetworkManager and getting an ip from
dhcp (the same one every time it asks courtesy dnsmasq configuration)

Any clues as to what is going on and what to adjust to make the machine
stay in the arp table and have it respond to arping and ping from
everywhere all the time?

Cheers
Hal