Network Assignment: NAT, ARP, Routing Protocols and TCP

NETWORK ASSIGNMENT
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QUESTION 1:
Overall Explanation
NAT is a property in a router which allow clients to access services through public IP address in
a network. This eliminates the need of having a separate domain name for a network
If a packet is sent from PC-1 (192.168.10.7/24) connected to the switch through port 1020, the
packet will be routed to the default gateway (192.168.10.1/24) of the router. A router with a
NAT loopback feature detects the incoming packet as 209.165.200.226/27 and is the interface of
its WAN network. It determines the destination of the packet depending on the port forwarding
rule. Data is then directed to the host address remote server that receives the packets. If the
DNAT rule is not applicable the router will drop the packet. An ICMP destination unreachable is
replied. If the rule is still there then the router rewrites the source IP address. The client
(192.168.10.7/24) sends the packets as incoming from (192.168.10.1/24) then the server receives
the packet as incoming from 209.165.200.226/27. When the request is replied by the server the
process is repeated. This process is identical for external senders. Thus this is a two way
communication.
Router entries
Network
destination
Net mask Gateway Interface Metric
192.168.10.7 255.255.255.0 192.168.10.1 209.165.200.226 10
192.168.10.9 255.255.255.0 192.168.10.1 209.165.200.226 10
Source and destination addresses
Source address: 192.168.10.7/24
Destination address: 209.165.200.226/27
Question 2:

1:
First PC-A (200.10.5.70/28) initiates the session by sending a broadcast ARP packet. The
address resolution packet is used in discovering the layer two address that is linked to layer three
addresses in the router. The broadcast then instructs the router to send the packet to all ports that
are connected to it. Computer PC-B (200.10.4.59/27) within layer three then responds with a
unicast ARP reply. The results are captured so that they can be looked up later. This makes the
ARP not to be sent frequently. This creates an ICMP request that is crafted. The source PC-A
(200.10.5.70/28) address will be originating computers’ ether address. The destination addresses
in the router and PC-B (200.10.4.59/27) will then be revealed from ARP. The packet is then sent
to the router through a cable Fa0/0 (200.10.5.70/28). The router then examines the destination
address of PC-B (200.10.4.59/27) and looks for it from the CAM table, this specifies the physical
port in the ether address. The packet is then transmitted through the wire to the correct physical
ports. When PC-B (200.10.4.59/27) computer receives the packet, it first examines the address
and makes sure that it is similar as that one in the physical interface. If the address is similar it
then checks if PC-B (200.10.4.59/27) matches with the address in layer 3 of the router as
assigned in the physical interface. If the packet matches, it is handled by upper layer protocols
(Pigeon, 2014) and if there is no match the packet is then dropped. For the ICMP reply, things
work in the same way, except that the responding computer doesn't need to send an ARP request.
The source computer receives reply from the connected computer if the pinging process is
successful.
2:
When PC-A (200.10.5.70/28) sends packet with the local network, PC-B (200.10.4.59/27) listens
to the incoming signal MAC address. If there is no answer within the network then the packet is
directed to the default gateway and configured in IP stack of the sending host. If the destination
address has not found any MAC address then the network is remote. If an address exits then the
network is local.