Question

Consider three LANs interconnected by two routers, as shown in Figure 6.33.

a. Assign IP addresses to all of the interfaces. For Subnet 1 use
addresses of the form 192.168.1.xxx; for Subnet 2 uses addresses of
the form 192.168.2.xxx; and for Subnet 3 use addresses of the form
192.168.3.xxx.
b. Assign MAC addresses to all of the adapters.
c. Consider sending an IP datagram from Host E to Host B. Suppose all of
the ARP tables are up to date. Enumerate all the steps, as done for the
single-router example in Section 6.4.1.
d. Repeat (c), now assuming that the ARP table in the sending host is empty
(and the other tables are up to date).

Answer 1

The answer provides assignments of IP and MAC addresses for the given network setup, explains the steps to send an IP datagram from Host E to Host B, and repeats the steps assuming an empty ARP table on Host E.

Step-by-step explanation:

a. Assigning IP addresses to all of the interfaces:

• Subnet 1: 192.168.1.1 (Router 1 Interface), 192.168.1.2 (Host A Interface)
• Subnet 2: 192.168.2.1 (Router 1 Interface), 192.168.2.2 (Router 2 Interface), 192.168.2.3 (Host B Interface)
• Subnet 3: 192.168.3.1 (Router 2 Interface), 192.168.3.2 (Host C Interface), 192.168.3.3 (Host D Interface), 192.168.3.4 (Host E Interface)

b. Assigning MAC addresses to all of the adapters:

• Host A: MAC1
• Host B: MAC2
• Host C: MAC3
• Host D: MAC4
• Host E: MAC5
• Router 1 Interface: MAC6
• Router 2 Interface: MAC7

c. Steps to send an IP datagram from Host E to Host B:

1. Host E checks if Host B is on the same subnet by comparing the destination IP address with its own subnet mask.
2. If Host B is on a different subnet, Host E sends the IP datagram to its default gateway (Router 2 Interface).
3. Router 2 checks its routing table and forwards the IP datagram to Router 1 Interface.
4. Router 1 checks its routing table and forwards the IP datagram to Host B.
5. Host B receives the IP datagram.

d. Steps to send an IP datagram from Host E to Host B with an empty ARP table on Host E:

1. Host E checks if Host B is on the same subnet by comparing the destination IP address with its own subnet mask.
2. Since Host E's ARP table is empty, Host E uses the Address Resolution Protocol (ARP) to discover the MAC address of its default gateway (Router 2 Interface).
3. Host E sends an ARP request to discover the MAC address of Router 2 Interface.
4. Router 2 Interface receives the ARP request and sends an ARP reply with its MAC address.
5. Host E now knows the MAC address of Router 2 Interface and sends the IP datagram to it.
6. Router 2 forwards the IP datagram to Router 1 Interface.
7. Router 1 forwards the IP datagram to Host B.
8. Host B receives the IP datagram.