Question

Consider three LANs interconnected by two routers, as shown in the figure below. 

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 5.4.1. 

Repeat(c), now assuming that the ARP table in the sending host is empty and the other tables are up to date).

Answer 1

(A) The network consists of 3 LAN (Local Area Networks) which are interconnected by 2 routers. The following are the range of IP (Internet Protocol) address for each subnet:

Subnet 1 use the address of the form

192.168.1.xxx

Subnet 2 use the address of the form 192.168.2.xxx

Subnet 3 use the address of the form

192.168.3.xxx

The following figure shows the assignment of IP address to the all the interfaces in the network:

192.168.1.001 00-00-00-00-00-00 192.168-2001 44-44-44-44-44-44 ' 192.168.3.001 77-77-77-77-77-770 M 3 1 AN 192.168.2.002 192.168.3.002 33-33-33-33-33-334 88-88-88-88-88-88 Router 1 (LAN router ? (LAN) 192.168.1.002 192.168.2.003 | 22–22-22-22-22-22 155-55-55-55-55-55 192.168.1.003 11-11-11-11-11-11 192.168.2.004 66-66-66-66-66-66 subnet 2 192.168.3.003 99-99-99-99-99-99 subnet 3 subnet a

(b) MAC addresses:

It is an address assigned to the hardware devices at the time of manufacturing. Assigning of MAC (Message Authentication Code) address to all the adapters in the network is shown in above network diagram.

(c) When an IP datagram is to be sent from Host

E to Host B with all the Address Resolution Protocol (ARP) updated tables, follow the

steps for the single router:

• The forwarding table in hostEL determines

that the datagram should be routed to interface 192.168.3.002

• Using the ARP table, adapter E determines the LAN address for 192.168.3.002 as 88-88-88-88-88-88

• The router 2 receives the packet with the Ethernet address as 88-88-88-88-88-88

• The forwarding table in the router 2 then indicates that the datagram is to be routed to

192.168.2.002 with destination address of

33-33-33-33-33-33 and the source address of

55-55-55-55-55-55 through the adapter IP

address 192.168.2.003.

• Router 1 receives the packet and extracts the datagram. The forwarding table in this router indicates that the packet should forwarded to 192.168.1.003.

• Therefore, the router 1 send the packet to the adapter B with the destination address of

11-11-11-11-11-11.

(*)

When an IP datagram is to be sent from Host E to Host B with sending host's ARP table is

empty, follow the steps:

• ARP table of the host Is empty, but, it should know the MAC address of the interface 192.168.3.002.

• Host E sends an ARP query packet within a broadcast. Then, all the interfaces and hosts

in the network receive the broadcast.

• Router 2 receives the query packet and send the ARP response packet.

• ARP response packet consists a MAC

address of the interface which is 88-88-88-88-88-88 with the destination address as 77-77-77-77-77-77.

. Now, the Host E knows the MAC address of router 2 and it updates its ARP table.

• Host E sends the packet to the interface

192.168.3.002

• Router 2 receives the packet and forwards it to the next router until the packet reaches to the Host B as all knows the ARP updated tables.