CCNA interview questions

Interview question 1

Two interfaces of the router is configured with IP addresses 192.168.1.1, subnet mask 255.255.255.0 and IP address 192.168.2.1, subnet mask 255.255.255.0. Would the routing table of the router contain any information? Provide explanations

Answer

When the interfaces are configured with the respective IP addresses and subnet mask, the router would perform an ANding operation with the IP address and subnet mask. This would ultimately yield the network address for the interface. Once the interfaces are configured with the IP addresses and subnet masks, two entries would be available in the routing tables which are 192.168.1.0 (Network address of 192.168.1.1) and 192.168.2.0 (Network address of 192.168.2.1).

Interview question 2

In the above diagram (in the book), three computers with IP address 192.168.1.2, 192.168.1.3 and 192.168.2.1 is connected to a layer 2 switch. Can the computer with IP address 192.168.1.2 ping the computer with IP address 192.168.2.1. Provide suitable explanation.

Answer

The computer with IP addresses 192.168.1.2 belongs to the network address of 192.168.1.0. The computer with IP address 192.168.2.1 belongs to the network address of 192.168.2.0. Since both the computers belong to different network addresses, communication between them is possible only if a layer 3 device or a router is available. Computers require the mac-address of the peer for sending out the packet. ARP is used in an ethernet segment to find out the mac-address of the peer. ARP can be used to find out the mac-addresses of devices belonging to the same network address. If the destination is on a different network address, the ARP request is sent to the gateway. So in this case, there is no gateway configured.

Interview question 3

A computer telnets to a router. Explain the communication process including details like packet and frame construction and TCP channel establishment for Telnet communication

Answer

The user types the command telnet 192.168.1.1 to initiate a connection to the router. The TCP/IP on the computer generates a TCP segment with a random source port and the destination port number as 23. Port 23 is selected since the user has used the command telnet and telnet servers are identified by TCP port 23. Once the TCP segment is created, the routing table of the computer checks the network address of the destination. It does this by performing an Anding operation with the destination IP address ( 192.168.1.1) and the local subnet mask of the computer. Since the result derives that both belong to the same network address, the computer checks in its ARP cache if the mac-address corresponding to the IP address is 192.168.1.1. If not, an ARP request is sent out on the network to discover the mac-address. Once the mac-address is discovered, the TCP segment with source port as random port and destination port as 23 and IP header with source IP address as 192.168.1.2 and destination IP address as 192.168.1.1 is encapsulated in an ethernet frame with source mac-address as computers and destination mac-address as routers mac-address. The TCP segment is the Syn segment which is used for initiating the TCP 3 way handshake with the telnet server on the router

Interview Question 4

The IP address of a PC is 192.168.1.2/24 and its default gateway is 192.168.1.1/24. The user on the PC pings an IP address which is on a different network (IP address: 192.168.2.2, Default gateway: 192.168.2.1). What would be the destination IP address in the IP header in the ping response packet coming from the IP address 192.168.2.2?

Explanation

The ping response packet would contain the initiator’s IP address in the destination IP address field in the IP header , which would be 192.168.1.2.

Interview Question 5

What is the use of a subnet mask in the routing table of a router?

Answer

The subnet mask is used to identify the network address corresponding to the received IP packets on the router. Assume that an IP packet has arrived with the destination IP address as 192.168.1.5.The route table contains the following entry.

192.168.1.0 255.255.255.0.

192.168.1.5 would be “And ed” with 255.255.255.0 and determined that the destination IP address is a member of the 192.168.1.0 network.

Interview Question 6

Which of the following fields in an ethernet frame would be modified when it moves from port 1 to port 2 of a switch?

-Destination mac-address
-Source mac-address
-Data

Answer

When an Ethernet frame is generated, the Destination mac-address would be the receiver’s mac-address and not the switch port’s address. When the frame passes through the port, none of the fields above would be modified.

Interview question 7

A router has two paths to a network with a static route and dynamic routing protocol. Which path would a packet bound to the network take when it reaches the router?

Answer

When there are two paths to the same network, the path with the lower administrative distance would be used for forwarding the packet. The AD (Administrative Distance) of static route is lower than that of dynamic routing protocols. So the path which the static route points to would be taken by the packet.