network segment range which NIDS can detect?

[SB CH]

Hello, all.

I installed snort NIDS at my linux which connected at switch and I 
confirmed that snort could detect some other servers were attacked. As I 
know, NIDS can detect some other attacks in the range of a network segment.
Then what is a "same network segment" in the switch?
I can detect some attacks to A server, but B isn't which connected with 
same switch.

Surely, I didnt' use the tab or span at switch.

Thanks in advance.

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