Machine A $($its IP address is $10\mathrm{.}9\mathrm{.}0\mathrm{.}5)$ inside an organization runs a TCP server using $"$nc $-$l $9090".$ Due to the $$rewall$,$ no outside machine can send data to this TCP server, except machine B $($its IP address is $192\mathrm{.}168\mathrm{.}60\mathrm{.}5).$ Namely, for a packet from the outside to reach this TCP server, the source IP address must be B$.$ You have noticed the following facts: $(1)$ machine A always uses a $$xed number $0$x$1000$ as its initial sequence number during the TCP three$-$way handshake protocol, and $(2)$ machine B is currently of$$ine$.$