Someone

No.     Time        Source                Destination           Protocol Info
      1 0.000000    172.17.101.56         Broadcast             ARP      Who has 172.17.100.172?  Tell 172.17.101.56

Frame 1 (60 bytes on wire, 60 bytes captured)
Ethernet II, Src: 00:d0:d9:06:55:78, Dst: ff:ff:ff:ff:ff:ff
Address Resolution Protocol (request)

No.     Time        Source                Destination           Protocol Info
      2 0.000034    172.17.100.172        172.17.101.56         ARP      172.17.100.172 is at 00:0b:6a:eb:36:df

Frame 2 (42 bytes on wire, 42 bytes captured)
Ethernet II, Src: 00:0b:6a:eb:36:df, Dst: 00:d0:d9:06:55:78
Address Resolution Protocol (reply)

No.     Time        Source                Destination           Protocol Info
      3 0.001684    labpc2.********    172.17.101.56         ARP      172.17.100.172 is at 00:d0:d9:05:a9:c3