I was having this exact same problem recently, yet, like you, all pcs were
accessing the internet fine.
When I double checked in zone alarm, the ip addresses of some of the
computers was different to the actual ip address, so it's no wonder they
couldn't be found.
I started by turning off zone alarm on all pc's, and  they could all see
each other.
Then, go to start/cmd/ipconfig /all  on each of the pcs to see what the ip
address is on them. Make sure you record the changes in the zone alarm
trusted zone. Now all of mine can see each other again even with zone alarm
running.
Hope this helps.

Michele Sayer


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