Top Most Interview Questions

Top Interview Questions:

• What is the primary similarity between a bridge and a router? What is the primary difference between a bridge and a router?

A: The primary similarity between a bridge and a router is that both devices increase the number of hosts that may be interconnected into a common communications network. The difference is that a bridge works by interconnecting separate segments of a single network, whereas a router interconnects separate networks.

• What is a packet? What is the primary similarity between a frame and a packet? What is the primary difference between a frame and a packet?

A: A packet is the means by which data is transported from one network to another. The similarity between a frame and a packet is that they both encapsulate data and provide an addressing scheme for delivering the data. The difference between a frame and a packet is that the frame delivers data between two devices sharing a common data link, whereas a packet delivers data across a logical pathway, or route, spanning multiple data links.

• As a packet progresses across an internetwork, does the source address change?

A: Neither the source nor the destination address of a packet changes as it progresses from the source of the packet to the destination.

• What is a network address? What is the purpose of each part of a network address?

A: Network addresses are the addresses used in packets. Each network address has a network part, which identifies a particular data link, and a host or node part, which identifies a specific device on the data link identified by the network part.

• What is the primary difference between a network address and a data link identifier?

A: A packet identifies a device from the perspective of the entire internetwork. A frame identifies a device from the perspective of a single data link. Because the connection between two devices across an internetwork is a logical path, a network address is a logical address. Because the connection between two devices across a data link is a physical path, a data link identifier is a physical address.

• What are the five layers of the TCP/IP protocol suite? What is the purpose of each layer?

A:    The five layers of the TCP/IP protocol suite are the following:

Physical layer

Data link layer

Internet (or IP) layer

Host-to-host layer

Application layer

• What is the most common IP version presently in use?

A: The most common IP version now in use is version 4.

• What is fragmentation? What fields of the IP header are used for fragmentation?

A: Routers perform fragmentation when a packet is longer than the maximum packet length (Maximum Transmission Unit, or MTU) supported by a data link onto which the packet must be transmitted. The data within the packet will be broken into fragments, and each fragment will be encapsulated in its own packet. The receiver uses the Identifier and Fragment Offset fields and the MF bit of the Flags field to reassemble the fragments.

• What is the purpose of the TTL field in the IP header? How does the TTL process work?

A: The Time to Live (TTL) field prevents “lost” packets from being passed endlessly through the IP internetwork. The field contains an 8-bit integer that is set by the originator of the packet. Each router through which the packet passes will decrement the integer by one. If a router decrements the TTL to zero, it will discard the packet and send an ICMP “time exceeded” error message to the packet’s source address.

• What is the first octet rule?

A: The first octet rule determines the class of an IP address as follows:

Class A: The first bit of the first octet is always 0.

Class B: The first two bits of the first octet are always 10.

Class C: The first three bits of the first octet are always 110.

Class D: The first four bits of the first octet are always 1110.

Class E: The first four bits of the first octet are always 1111.

• How are class A, B, and C IP addresses recognized in dotted decimal? How are they recognized inbinary?

A: The A, B, C IP addresses are recognized in dotted decimal and binary as follows:

Class Binary Range of First Octet Decimal Range of First Octet

A 0000000 – 01111110 1 – 126

B 10000000 – 10111111 128 – 191

C 11000000 – 11011111 192- 223

• What is an address mask, and how does it work?

A: An IP address mask identifies the network part of an IP address. Each one in the 32-bit mask marks the corresponding bit in the IP address as a network bit. A zero in the mask marks the

Corresponding bit in the IP address as a host bit. A Boolean AND is performed in all 32 bits of the address and the mask; in the result, all network bits of the mask will be repeated, and all host bits will be changed to zero.

• What is a subnet? Why are subnets used in IP environments?

A: A subnet is a sub grouping of a class A, B, or C IP address. Without subletting, the network part of a major class A, B, or C IP address can only identify a single data link. Subnetting uses some of the host bits of a major IP address as network bits, allowing the single major address to be “Subdivided” into multiple network addresses.

• Why can’t a subnet of all zeros or all ones be used in a classful routing environment?

A: A classful routing protocol has no way to differentiate between the all-zeroes subnet and the major IP address, and between the all-ones subnet and the all-hosts, all-subnets broadcast address of the major IP address.

• What is ARP?

A: ARP, or Address Resolution Protocol, is a function that maps the IP addresses of interfaces on a data link to their corresponding MAC identifiers.

• What is proxy ARP?

A: Proxy ARP is a function of an IP router. If the router hears an ARP request, and

The destination network or subnet is in the router’s routing table, and

The table indicates that the destination is reachable via a different router interface than the one on which the ARP request was received, The router will respond to the ARP request with its own MAC address.