Below is an example of the % reachability for several SLAC foreign collaborator hosts for Jan-Oct-96. The reachability is calculated from the pinging data. Each host is pinged 21 times at 30 minute intervals. If all pings fail to respond the host is marked as down. Then
reachability = # downs / # intervals
You can also look at an animated reachability display for the groups of Internet hosts that SLAC pings.
The unreachability (= 1 - reachability) averaged over the hosts in each group is seen below.
We can also keep track of the number of intervals that the hosts were marked as down, see below for an example of the number of downs seen from SLAC for SLAC N. American collaborator nodes: