Question $12$

Consider the image below in which three hosts, with private IP addresses $10\mathrm{.}0\mathrm{.}1\mathrm{.}14,10\mathrm{.}0\mathrm{.}1\mathrm{.}15,10\mathrm{.}0\mathrm{.}1\mathrm{.}23$ are in a local network behind a NAT'ted router that sits between these three hosts and the larger Internet. IP datagrams being sent from, or destined to$,$ these three hosts must pass through this NAT router. The router's interface on the LAN side has IP address $10\mathrm{.}0\mathrm{.}1\mathrm{.}25,$ while the router's address on the Internet side has IP address $135\mathrm{.}122\mathrm{.}193\mathrm{.}215.$

Suppose that the host with IP address $10\mathrm{.}0\mathrm{.}1\mathrm{.}14$ sends an IP datagram destined to host $128\mathrm{.}119\mathrm{.}173\mathrm{.}182.$ The source port is $3391,$ and the destination port is $80.$

$1.$ Consider the datagram at step $1,$ after it has been sent by the host but before it has reached the NATted router. $(2$ pts $)$

$1.$ What are the source and destination IP addresses for this datagram? Suppose that the host with IP address $10\mathrm{.}0\mathrm{.}1\mathrm{.}14$ sends an IP datagram destined to host $128\mathrm{.}119\mathrm{.}173\mathrm{.}182.$ The source port is $3391,$ and the destination port is $80.$

$1.$ Consider the datagram at step $1,$ after it has been sent by the host but before it has reached the NATted router. $(2$ pts$)$

$1.$ What are the source and destination IP addresses for this datagram?

$2.$ What are the source and destination port numbers for the TCP segment in this IP datagram?

$2.$ Now consider the datagram at step $2,$ after it has been transmitted by the NATted router. $(4$ pts $)$

$1.$ What are the source and destination IP addresses for this datagram?

$2.$ What are the source and destination port numbers for the TCP segment in this IP datagram? Assume the NAT'ed port number is $500$

$3.$ Specify the entry that has been made in the router's NAT table.

$3.$ Now consider the datagram at step $3,$ just before it is received by the NATted router. $(2$ pts$)$

$1.$ What are the source and destination IP addresses for this datagram?

$2.$ What are the source and destination port numbers for the TCP segment in this IP datagram?

$4.$ Last, consider the datagram at step $4,$ after it has been transmitted by the NATted router but before it has been received by the host. $(4$ pts$)$

$1.$ What are the source and destination IP address for this datagram?

$2.$ What are the source and destination port numbers for the TCP segment in this IP datagram

$3.$ Has a new entry been made in the router's NAT table, or removed from the NAT table?

$4.$ Explain your answer to $4\mathrm{.}4$ above