Offline nodes appear in the console

There are 2 main reasons a node may appear as "offline" 

1. Decommissioned node- a decommissioned node will show as offline for 4 days by default before the other running nodes forget about it. Rebooted nodes won't remember the offline node.  This is controlled by the crier.forgetOfflineInterval parameter (345600 - in seconds- by default).
2. IGMP snooping- a very common reason to find a node as offline, especially if the node has been online in the past, is that IGMP snooping has been enabled in the layer 2 switch infrastructure.    This feature blocks multicasts and has absolutely no benefit in the Swarm private subnet.  It should be disabled at the switch.  See here: IGMP Snooping