What is a LAN, and what is a WAN?

A LAN (local area network) is the network inside one building or site, connecting its computers, printers and servers. A WAN (wide area network) connects networks across distances, for example several offices of the same company or a site to the internet. Traffic inside the LAN often never passes the firewall at the internet edge, so a firewall alone does not see a compromised PC attacking the file server next to it. That internal traffic, sometimes called east-west traffic, is what network detection on a mirror port watches. Links between sites, often VPNs, deserve the same attention.

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