The attacker is on the same subnet as the target system.
Variants:
1) The attacker could send Gratuitous ARP (GARP) to clam that the attacker’s Layer 2 MAC address is the MAC address of the nex-hop router. So, the attacker would capture all the traffic and forward it to the legitimate next-hop router.
2) The attacker can connect a hub to the network segment that carries the traffic the attacker wants to capture.
3)The attacker could connect to a Switch Port Analyzer (SPAN) port to capture all the traffic.
