Top Interview Questions for Network Engineers, Part 1

This was originally posted by another blog, router-switch.com and I must say that agree only partially. Some questions are too deep, but not related to Networks exactly, but more to the Systems part, while the purely Networking questions are not so deep. No-one who´s looking for an actual Network Engineer will ask you "What is OSPF?" Still, I enjoyed browsing through it, and it might be true - Network/Systems engineer should know all this...


Q1. What are 10Base2, 10Base5 and 10BaseT Ethernet LANs?
Ans. 10Base2—An Ethernet term meaning a maximum transfer rate of 10 Megabits per second that uses baseband signaling, with a contiguous cable segment length of 100 meters and a maximum of 2 segments.


10Base5—An Ethernet term meaning a maximum transfer rate of 10 Megabits per second that uses baseband signaling, with 5 continuous segments not exceeding 100 meters per segment.

10BaseT—An Ethernet term meaning a maximum transfer rate of 10 Megabits per second that uses baseband signaling and twisted pair cabling.



Q2. What is the difference between an unspecified passive open and a fully specified passive open?

Ans. An unspecified passive open has the server waiting for a connection request from a client. A fully specified passive open has the server waiting for a connection from a specific client.


Q3. Explain the function of Transmission Control Block.

Ans. A TCB is a complex data structure that contains a considerable amount of information about each connection.


Q4. What is a Management Information Base (MIB)?

Ans. A Management Information Base is part of every SNMP-managed device. Each SNMP agent has the MIB database that contains information about the device’s status, its performance, connections, and configuration. The MIB is queried by SNMP.


Q5. What is anonymous FTP and why would you use it?

Ans. Anonymous FTP enables users to connect to a host without using a valid login and password. Usually, anonymous FTP uses a login called anonymous or guest, with the password usually requesting the user’s ID for tracking purposes only. Anonymous FTP is used to enable a large number of users to access files on the host without having to go to the trouble of setting up logins for them all. Anonymous FTP systems usually have strict controls over the areas an anonymous user can access.


Q6. What is a pseudo tty?

Ans. A pseudo tty or false terminal enables external machines to connect through Telnet or rlogin. Without a pseudo tty, no connection can take place.


Q7. Which layer of the 7 layer model provides services to the Application layer over the Session layer connection?

Ans. Presentation.


Q8. What does the Mount protocol do?

Ans. The Mount protocol returns a file handle and the name of the file system in which a requested file resides. The message is sent to the client from the server after reception of aclient’s request.


Q9. What is External Data Representation?

Ans. External Data Representation is a method of encoding data within an RPC message, used to ensure that the data is not system-dependent.


Q10. Which OSI Reference Layer controls application to application communication?

Ans. Session


Q11. BOOTP helps a diskless workstation boot. How does it get a message to the network looking for its IP address and the location of its operating system boot files ?

Ans. BOOTP sends a UDP message with a subnetwork broadcast address and waits for a reply from a server that gives it the IP address. The same message might contain the name of the machine that has the boot files on it. If the boot image location is not specified, the workstation sends another UDP message to query the server.


Q12. What is a DNS resource record?

Ans. A resource record is an entry in a name server’s database. There are several types of resource records used, including name-to-address resolution information. Resource records are maintained as ASCII files.


Q13. What protocol is used by DNS name servers?

Ans. DNS uses UDP for communication between servers. It is a better choice than TCP because of the improved speed a connectionless protocol offers. Of course, transmission reliability suffers with UDP.


Q14. What is the difference between interior and exterior neighbor gateways?

Ans. Interior gateways connect LANs of one organization, whereas exterior gateways connect the organization to the outside world.


Q15. What is the HELLO protocol used for?

Ans. The HELLO protocol uses time instead of distance to determine optimal routing. It is an alternative to the Routing Information Protocol.


Q16. What are the advantages and disadvantages of the three types of routing tables?

Ans. The three types of routing tables are fixed, dynamic, and fixed central. The fixed table must be manually modified every time there is a change. A dynamic table changes its information based on network traffic, reducing the amount of manual maintenance. A fixed central table lets a manager modify only one table, which is then read by other devices. The fixed central table reduces the need to update each machine’s table, as with the fixed table. Usually a dynamic table causes the fewest problems for a network administrator, although the table’s contents can change without the administrator being aware of the change.


Q17. What is a characteristic of Store and Forward switches?

Ans. They read the entire frame and check CRC before forwarding.


Q18. What is source route?

Ans. It is a sequence of IP addresses identifying the route a datagram must follow. A source route may optionally be included in an IP datagram header.


Q19. What is RIP (Routing Information Protocol)?

Ans. It is a simple protocol used to exchange information between the routers.


Q20. What is SLIP (Serial Line Interface Protocol)?

Ans. It is a very simple protocol used for transmission of IP datagrams across a serial line.


Q21. What is Proxy ARP?

Ans. It is using a router to answer ARP requests. This will be done when the originating host believes that a destination is local, when in fact is lies beyond router.


Q22. What is OSPF?

Ans. It is an Internet routing protocol that scales well, can route traffic along multiple paths, and uses knowledge of an Internet’s topology to make accurate routing decisions.


Q23. What is Kerberos?

Ans. It is an authentication service developed at the Massachusetts Institute of Technology. Kerberos uses encryption to prevent intruders from discovering passwords and gaining unauthorized access to files.


Q24. What is a Multi-homed Host?

Ans. It is a host that has a multiple network interfaces and that requires multiple IP addressesis called as a Multi-homed Host.


Q25. What is NVT (Network Virtual Terminal)?

Ans. It is a set of rules defining a very simple virtual terminal interaction. The NVT is used in the start of a Telnet session.


Q26. What is Gateway-to-Gateway protocol?

Ans. It is a protocol formerly

used to exchange routing information between Internet core routers.


Q27. What is BGP (Border Gateway Protocol)?

Ans. It is a protocol used to advertise the set of networks that can be reached with in an autonomous system. BGP enables this information to be shared with the autonomous system. This is newer than EGP (Exterior Gateway Protocol).


Q28. What is autonomous system?

Ans. It is a collection of routers under the control of a single administrative authority and that uses a common Interior Gateway Protocol.


Q29. What is EGP (Exterior Gateway Protocol)?

Ans. It is the protocol the routers in neighboring autonomous systems use to identify the set of networks that can be reached within or via each autonomous system.


Q30. What is IGP (Interior Gateway Protocol)?

Ans. It is any routing protocol used within an autonomous system.


Q31. What is Mail Gateway?

Ans. It is a system that performs a protocol translation between different electronic mail deliveryprotocols.


Q32. What is wide-mouth frog?

Ans. Wide-mouth frog is the simplest known key distribution center (KDC) authentication protocol.


Q33. What are Digrams and Trigrams?

Ans. The most common two letter combinations are called as digrams. e.g. th, in, er, re and an. The most common three letter combinations are called as trigrams. e.g. the, ing, and, and ion.


Q34. What is silly window syndrome?

Ans. It is a problem that can ruin TCP performance. This problem occurs when data are passed to the sending TCP entity in large blocks, but an interactive application on the receiving side reads 1 byte at a time.


Q35. What is region?

Ans. When hierarchical routing is used, the routers are divided into what we call regions, with each router knowing all the details about how to route packets to destinations within its own region, but knowing nothing about the internal structure of other regions.


Q36. What is multicast routing?

Ans. Sending a message to a group is called multicasting, and its routing algorithm is called multicast routing.


Q37. What is traffic shaping?

Ans. One of the main causes of congestion is that traffic is often busy. If hosts could be made to transmit at a uniform rate,congestion would be less common. Another open loop method to help manage congestion is forcing the packet to be transmitted at a more predictable rate. This is called traffic shaping.


Q38. What is packet filter?

Ans. Packet filter is a standard router equipped with some extra functionality. The extra functionality allows every incoming or outgoing packet to be inspected. Packets meeting some criterion are forwarded normally. Those that fail the test are dropped.


Q39. What is virtual path?

Ans. Along any transmission path from a given source to a given destination, a group of virtual circuits can be grouped together into what is called path.


Q40. What is virtual channel?

Ans. Virtual channel is normally a connection from one source to one destination, although multicast connections are also permitted. The other name for virtual channel is virtual circuit.


Q41. What is logical link control?

Ans. One of two sublayers of the data link layer of OSI reference model, as defined by the IEEE 802 standard. This sublayer is responsible for maintaining the link between computers when they are sending data across the physical network connection.


Q42. Why should you care about the OSI Reference Model?

Ans. It provides a framework for discussing network operations and design.


Q43. What is the difference between routable and non- routable protocols?

Ans. Routable protocols can work with a router and can be used to build large networks. Non-Routable protocols are designed to work on small, local networks and cannot be used with a router.


Q44. What is MAU?

Ans. In token Ring , hub is called Multistation Access Unit(MAU).


Q45. Explain 5-4-3 rule?

Ans. In a Ethernet network, between any two points on the network, there can be no more than five network segments or four repeaters, and of those five segments only three of segments can be populated.


Q46. What is the difference between TFTP and FTP application layer protocols?

Ans. The Trivial File Transfer Protocol (TFTP) allows a local host to obtain files from a remote host but does not provide reliability or security. It uses the fundamental packet delivery services offered by UDP. The File Transfer Protocol (FTP) is the standard mechanism provided by TCP / IP for copying a file from one host to another. It uses the services offered by TCP and so is reliable and secure. It establishes two connections (virtual circuits) between the hosts, one for data transfer and another for control information.


Q47. What is the range of addresses in the classes of internet addresses?

Ans. Class A 0.0.0.0 – 127.255.255.255

Class B 128.0.0.0 – 191.255.255.255

Class C 192.0.0.0 – 223.255.255.255

Class D 224.0.0.0 – 239.255.255.255

Class E 240.0.0.0 – 247.255.255.255


Q48. What is the minimum and maximum length of the header in the TCP segment and IP datagram?

Ans. The header should have a minimum length of 20 bytes and can have a maximum length of 60 bytes.


Q49. What is difference between ARP and RARP?

Ans. The address resolution protocol (ARP) is used to associate the 32 bit IP address with the 48 bit physical address, used by a host or a router to find the physical address of another host on its network by sending a ARP query packet that includes the IP address of the receiver. The reverse address resolution protocol (RARP) allows a host to discover its Internet address when it knows only its physical address.


Q50. What is ICMP?

Ans. ICMP is Internet Control Message Protocol, a network layer protocol of the TCP/IP suite used by hosts and gateways to send notification of datagram problems back to the sender. It uses the echo test / reply to test whether a destination is reachable and responding. It also handles both control and error messages.