This is a numeric identifier for the node used specifically in X.25 routing to prevent routing loops across the network. Because this feeds directly into the loop-prevention logic of the X.25 routing protocol, an admin must ensure it's unique across the entire multi-node network; a duplicate node number on two nodes could cause the routing protocol's loop-detection to misbehave. For example, assigning node number 14 to a newly added branch node requires first confirming no other node in the network already uses 14, or X.25 call routing could become unreliable.