Network Fundamentals Interview Questions - Computer Networks

Network fundamentals cover the core principles that enable devices to communicate and share data. They include networking models like OSI and TCP/IP, addressing schemes such as IP and MAC. Understanding these basics is essential for troubleshooting, designing, and optimizing modern communication systems, making them a foundation for advanced networking discussions and interviews.

1. If two computers are connected directly using a cross-over cable, can they communicate without IP addresses? Why or why not?

• Yes, they can communicate at the data link layer using MAC addresses, even without IP addresses.
• The network interface card (NIC) recognizes frames by destination MAC. If one computer knows the MAC of the other, frames can be exchanged.
• However, without IP addresses, they cannot communicate at the network layer (no routing, no logical addressing). Protocols like HTTP, FTP (which depend on IP) will fail.
• Practical communication (like file sharing) usually requires IP, but raw frame-level communication (like ARP, custom Ethernet frames) is possible without IP.

2. What are the different classes of IP addresses and give the range of each class?

IP address is an address having information about how to reach a specific host, especially outside the LAN. An IP address is  32-bit unique address having an address space of 232.

For more details, please refer introduction of Classful IP addressing.

3. What is subnet mask?

A subnet mask is a 32-bit number that is used to identify the subnet of an IP address. The subnet mask is a combination of 1’s and 0’s. 1’s represents network and subnet ID while 0’s represents the host ID. In this case, the subnet mask is,

11111111.11111111.11111111.11000000
or
255.255.255.192

So, in order to get the network to which the destination address belongs, we have to bitwise & with a subnet mask.

11111111.11111111.11111111.11000000
&& 11001000.00000001.00000010.00010100
-----------------------------------------------------
11001000.00000001.00000010.00000000

The address belongs to,

11001000.00000001.00000010.00000000
or
200.1.2.0

4. Why CIDR is used?

The problem with this classful addressing method is that millions of class A addresses are wasted, many of the class B addresses are wasted, whereas, number of addresses available in class C is so small that it cannot cater to the needs of organizations. Class D addresses are used for multicast routing and are therefore available as a single block only. Class E addresses are reserved. Since there are these problems, Classful networking was replaced by Classless Inter-Domain Routing (CIDR).

For more details please refer CIDR full-form to the article.

5. What is the LOOPBACK address?

Loopback Address is used to let a system send a message to itself to make sure that the TCP/IP stack is installed correctly on the machine.

For more details, please refer Local Broadcast and Loopback address article.

6. What is a Default Gateway?

A Default Gateway is the device (usually a router) that connects your local network to other networks, including the internet.

• When a computer in a local network wants to send data to an IP address outside its own network, it doesn’t know where to send it directly.
• In this case, the data is forwarded to the default gateway, which then routes it towards the correct destination.

Example:

• Your PC’s IP: 192.168.1.10
• Your network’s default gateway: 192.168.1.1 (the router)
  If you try to access 8.8.8.8 (Google DNS), your PC sends the packet to the gateway (192.168.1.1), and the router forwards it to the internet.

7.  Why Hop limit field is used?

The Hop Limit field (in IPv6, similar to TTL in IPv4) is used to prevent packets from circulating endlessly in case of routing loops.

• Each time a packet passes through a router (a “hop”), the hop limit value is decremented by 1.
• If it reaches 0, the packet is discarded.

For more details, please refer to Internet Protocol version 6 (IPv6) Header article.

8. What protocol is used by PING?

PING uses the ICMP (Internet Control Message Protocol).

• It sends an ICMP Echo Request to the target host.
• If the host is reachable, it replies with an ICMP Echo Reply.

For more details, please refer to the Difference between Ping and Traceroute article.

9. What is used of Tracert?

Tracert (Windows) or traceroute (Linux) is a network diagnostic tool used to trace the path packets take from your computer to a destination host.

• It shows each hop (router) along the path and the time taken to reach it.
• Useful for identifying where delays, failures, or routing issues occur in a network.

For more details, please refer to the Difference between Ping and Traceroute article.

10. Name the ports used by FTP protocol?

FTP protocol uses two ports:

• Control connection: For sending control information like user identification, password, commands to change the remote directory, commands to retrieve and store files, etc., FTP makes use of connections. The control connection is initiated on port number 21.
• Data connection: For sending the actual file, FTP makes use of a data connection. A data connection is initiated on port number 20.

For more details, please refer to File Transfer Protocol (FTP) in the Application Layer article.

11. What is MAC address?

MAC Addresses are unique 48-bit hardware numbers of computers, which are embedded into a network card (known as Network Interface Card) during the time of manufacturing. The MAC Address is also known as the Physical Address of a network device. In IEEE 802 standard, the Data Link Layer is divided into two sublayers – 

1. Logical Link Control(LLC) Sublayer
2. Media Access Control(MAC) Sublayer

The MAC address is used by the Media Access Control (MAC) sublayer of the Data-Link Layer. MAC Address is unique worldwide since millions of network devices exist and we need to uniquely identify each. 

For more details, please refer to Introduction of MAC Address in Computer Network article.

12. Explain ARP?

ARP (Address Resolution Protocol) is a network protocol used to map an IP address (logical address) to a MAC address (physical address) within a local network.

• When a computer wants to communicate with another device on the same LAN, it only knows the IP address.
• ARP sends a broadcast request asking: “Who has this IP?”
• The device with that IP replies with its MAC address, which is then stored in the ARP cache for faster future communication.

For more details, please refer to the How Address Resolution Protocol works article.

13.  What is MTU?

A maximum transmission unit also called MTU, is a term used in networking and operating systems. It defines the largest size of the packet that can be transmitted as a single entity in a network connection. The size of the MTU dictates the amount of data that can be transmitted in bytes over a network.

For more details, please refer What is MTU article.

14. If a class B network on the Internet has a subnet mask of 255.255.248.0, what is the maximum number of hosts per subnet?

• Subnet mask: 255.255.248.0 → Binary: 11111111.11111111.11111000.00000000
• Total network bits = 21 → Host bits = 32 − 21 = 11
• Total host addresses = 2^11 = 2048
• Reserved addresses:
  • All 0s → Network address
  • All 1s → Broadcast address
• Usable host addresses = 2048 − 2 = 2046

Final Answer: 2046 hosts per subnet

15. What is IP multicast?

IP Multicast is a communication method where data is sent from one source to multiple receivers at once using a single transmission.

• Instead of sending separate copies to each receiver (like unicast), multicast delivers packets to a multicast group address.
• Devices that want the data “join” the multicast group using protocols like IGMP (IPv4) or MLD (IPv6).
• Efficient for streaming video, online gaming, and real-time data feeds, since it saves bandwidth.

16. Difference between public and private IP addresses?

1. Public IP address

• A globally unique IP address assigned by your ISP and used to identify your device or network on the internet.
• Works like a postal address it’s how other servers (web, email, etc.) find and communicate with you.
• Also called external IP.
• Example: 8.8.8.8 (Google DNS).

2. Private IP address

• Used inside a local network (LAN) to identify devices like laptops, phones, smart TVs, and printers.
• Assigned by your router, not visible directly on the internet.
• Multiple networks can reuse the same private IP ranges.
• Example ranges: 192.168.x.x, 10.x.x.x, 172.16.x.x – 172.31.x.x.

18. What is Network Address Translation?

• NAT allows multiple devices in a private network to access the internet using a single public IP address.
• It translates private IPs ↔ public IPs and also modifies port numbers to keep connections unique.
• The mapping of private IP/port to public IP/port is stored in a NAT table.
• NAT is usually implemented on routers or firewalls.

For more details, please refer to Network Address Translation.

19. An organization requires a range of IP addresses to assign one to each of its 1500 computers. The organization has approached an Internet Service Provider (ISP) for this task. The ISP uses CIDR and serves the requests from the available IP address space 202.61.0.0/17. The ISP wants to assign address space to the organization which will minimize the number of routing entries in the ISP’s router using route aggregation. To calculate the address spaces are potential candidates from which the ISP can allow any one of the organizations?

Subnet Mask for the given IP address:

202.61.0.0/17
⇒ 11111111 11111111 10000000 00000000
⇒ 255.255.128.0

Now, since we need 1500 hosts, so, bits for host address,

= ceiling (log2 (1500))
= ceiling (10.55)
= 11 bits for host address

So, the last 11 bits will be for host addresses:

00000000.00000000 → 00000111.11111111 (0.0 → 7.255)

00001000.00000000 → 00010000.00000000 (8.0 - 15.255)

00001111.11111111 → 00010111.11111111 (16.0 - 23.255)

Sequences are 0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 96, 104, 112, 120.

Hence, 64 and 104 are present in the sequence, so 202.61.104.0 / 21 and 202.61.64.0 / 21 are the possible IP addresses.

20. Explain the difference between Static and Dynamic IP?

For more details, please refer to Difference between Static and Dynamic IP addresses.

21. In a switched Ethernet network, if two hosts send frames to the same destination MAC address simultaneously, will there be a collision? Why or why not?

No, there will be no collision at the MAC layer in a switched Ethernet network. Here’s why:

• In older hub-based Ethernet (shared medium), all devices shared the same collision domain, so simultaneous transmissions caused collisions.
• In a switch-based network, each port represents a separate collision domain. The switch receives frames on different input ports and forwards them to the output port corresponding to the destination MAC.
• If two frames arrive for the same destination at the same time, the switch queues them internally and transmits sequentially, avoiding physical collisions.
• However, congestion may occur if the output port is overloaded, leading to buffering delays or packet drops, but not collisions.

22. What are the features of Gateway?

Gateways provide a wide variety of features. Some of which are:

• Gateways work as a network bridge for data transmission as it makes the transmission of data possible to transmit with more ease and does not demand high storage capacity.
• Gateways create a structural temporary storeroom for the data transmitted by the server and data requests made by the user end.
• Gateways made the transmission more feasible as it queued up all the data and divide it into small packets of data rather than sending it bulk. Data transmitted through Gateway is divided into various useful and small packets each having its individual significance and a role to play while processing data.
• Gateways made the data more secure if the modifications to the gateway could be done which then could create more reliability over smart devices.
• Gateways optimize the data for search engines, applications, and servers by implanting better readability to the content so that a machine could understand and optimize data with ease. For more details, please refer to the Introduction of Gateways.

23. Is Ipv6 Backward Compatible With Ipv4?

No, IPv6 is not backward compatible with IPv4 protocol.  

For more details, please refer to the Internet protocol version 6 article.

24. Is It Possible To Have An Ipv4 And An Ipv6 Addresses Simultaneously?

Yes, it is possible to have an IPv4 and IPv6 addresses simultaneously.

For more details, please refer to the Internet protocol version 6  and the difference between IPv4 and IPv6 articles.

25. What is TTL?

The lifespan or lifetime of data that is being sent. Once after that specified time is over or elapsed, the data will be discarded Or it can also be stated as the number of hops that packet is set to exist in the network, after which that packet is discarded. The purpose of the TTL field is to avoid a situation in which an undeliverable datagram keeps circulating in the network.

For more details, please refer to the difference between RTT and TTL article.

26. If the TTL field has the value of 10. How many routers (max) can process this datagram?

TTL stands for Time to Live. This field specifies the life of the IP packet based on the number of hops it makes ( number of routers it goes through). TTL field is decremented by one each time the datagram is processed by a router. When the value is 0, the packet is automatically destroyed.

For more details, please refer to the difference between RTT and TTL articles.

27. If the value in the protocol field is 17, the transport layer protocol used is which protocol?

If the value in the protocol field is 17, the transport layer protocol uses UDP (User Datagram Protocol).

For more details, please refer to UDP article.

28. What happens in  classless addressing, if there are no classes but addresses are still granted?

In classless addressing, there are no classes but addresses are still granted in blocks. The total number of addresses in a block of classless IP addresses = 2(32 – CIDR_value).  

For more details, please refer to the introduction of Classful IP addressing.

29.  Suppose two IPv6 nodes want to interoperate using IPv6 datagrams, but they are connected to each other by intervening IPv4 routers. Then what is the best solution?

If two IPv6 nodes want to interoperate using IPv6 datagrams, they are connected to each other by intervening IPv4 routers. Then tunneling is the best solution.  

For more details, please refer to  Internet protocol version 6 article.

30. What is IANA?

IANA, the Internet Assigned Numbers Authority, is an administrative function of the Internet that keeps track of IP addresses, domain names, and protocol parameter identifiers that are used by Internet standards. Some of these identifiers are parameters, such as those used by Internet protocols (like TCP, ICMP or UDP) to specify functions and behaviour; some of them represent Internet addresses and others represent domain names. Regardless of the type of identifier, the IANA function (IANA for short below) ensures that values are managed for uniqueness and made available in publicly accessible registries.

31. What is DHCP?

DHCP is an abbreviation for Dynamic Host Configuration Protocol. It is an application layer protocol used by hosts for obtaining network setup information. The DHCP is controlled by a DHCP server that dynamically distributes network configuration parameters such as IP  addresses, subnet mask, and gateway address.

For more details, please refer to Dynamic Host Configuration Protocol (DHCP) article.

32. How can you manage a network using a router?

Routers have built-in console that lets you configure different settings, like security and data logging.We can assign restrictions to computers, such as what resources they are allowed to access, or what particular time of the day they can browse the internet.We can even put restrictions on what websites are not viewable across the entire network.

For more details, please refer to the introduction of a Router.

33. What is ipconfig?

IPCONFIG stands for Internet Protocol Configuration. This is a command-line application that displays all the current TCP/IP (Transmission Control Protocol/Internet Protocol) network configuration, refreshes the DHCP (Dynamic Host Configuration Protocol) and DNS (Domain Name Server). It also displays an IP address, subnet mask, and a default gateway for all adapters. It is available for Microsoft Windows, ReactOS, and Apple macOS. ReactOS version was developed by Ged Murphy and licensed under the General Public License.

For more details, please refer to ipconfig full form article.

34. When you move the NIC cards from one PC to another PC, does the MAC address get transferred as well?

Yes, if we move the NIC cards from one PC to another PC, then the MAC address also gets transferred, because the MAC address is hard-wired into the NIC circuit, not the personal computer. This also means that a PC can have a different MAC address when another one replaces the NIC card. 

35. Explain clustering support?

Clustering support refers to the ability of a network operating system to connect multiple servers in a fault-tolerant group. The main purpose of this is that in the event that one server fails, all processing will continue on with the next server in the cluster.

36. What is Brouter?

It is also known as the bridging router is a device that combines features of both bridge and router. It can work either at the data link layer or a network layer. Working as a router, it is capable of routing packets across networks, and working as the bridge, it is capable of filtering local area network traffic.

For more details, please refer to the difference between Router and Brouter article.

37. Explain the features of VPN?

1. VPN also ensures security by providing an encrypted tunnel between client and vpn server.
2. VPN is used to bypass many blocked sites.
3. VPN facilitates anonymous browsing by hiding your ip address.
4. Also, the most appropriate Search engine optimization(SEO) is done by analyzing the data from VPN providers which provide country-wise states for browsing a particular product. This method of SEO is used widely by many internet marketing managers to form new strategies For more details, please refer to Virtual Private Network.

38. What are the important differences between MAC address and IP address?

39. What is 127.0.0.1?

In IPv4, IP addresses that start with decimal 127 or that has 01111111 in the first octet are loopback addresses(127.X.X.X). Typically 127.0.0.1 is used as the local loopback address.
This leads to the wastage of many potential IP addresses. But in IPv6 ::1 is used as local loopback address and therefore there isn’t any wastage of addresses.

40. What is a DNS?

DNS is a host name to IP address translation service. DNS is a distributed database implemented in a hierarchy of name servers. It is an application layer protocol for message exchange between clients and servers.

For more details, please refer to DNS in the application layer.

41. What is the use of a proxy server?

Proxy server refers to a server that acts as an intermediary between the request made by clients, and a particular server for some services or requests for some resources. There are different types of proxy servers available that are put into use according to the purpose of a request made by the clients to the servers. The basic purpose of Proxy servers is to protect the direct connection of Internet clients and internet resources. The proxy server also prevents the identification of the client’s IP address when the client makes any request is made to any other servers.

42. What is the difference between ipconfig and ifconfig commands?

IPCONFIG (Windows)

• Stands for Internet Protocol Configuration.
• Command-line tool to view and manage TCP/IP configuration.
• Displays IP address, subnet mask, default gateway, DNS, and DHCP details.
• Commonly used to refresh DHCP leases or flush DNS cache.
• Available on Windows, ReactOS, and macOS.

IFCONFIG (Linux/Unix)

• Stands for Interface Configuration.
• Used to configure and manage network interfaces.
• Can assign IP addresses, netmasks, and enable/disable interfaces.
• Often used for debugging or tuning network settings.
• Modern Linux distributions prefer the ip command instead of ifconfig.

43. What is the importance of APIPA in networking?

• APIPA is a feature in Windows that automatically assigns an IP address from the range 169.254.0.1 – 169.254.255.254 when a system fails to get an IP from a DHCP server.
• It allows devices on the same local network to still communicate with each other, even without DHCP.
• However, APIPA does not provide internet access, only limited LAN communication.

44. What is the difference between Firewall and Antivirus?

45. What is SLIP?

• SLIP is an old protocol used to transmit IP packets over serial connections (like modems).
• It is very simple and lightweight, but supports only IP (no error detection, no support for multiple protocols).
• Each IP packet is separated by a special END character (0xC0).
• Today, SLIP is obsolete and has been replaced by more advanced protocols like PPP (Point-to-Point Protocol).

46.  What is Kerberos protocol?

Kerberos provides a centralized authentication server whose function is to authenticate users to servers and servers to users. In Kerberos Authentication server and database is used for client authentication. Kerberos runs as a third-party trusted server known as the Key Distribution Center (KDC). Each user and service on the network is a principal.

The main components of Kerberos are:

• Authentication Server (AS): The Authentication Server performs the initial authentication and ticket for Ticket Granting Service.
• Database: The Authentication Server verifies the access rights of users in the database.
• Ticket Granting Server (TGS): The Ticket Granting Server issues the ticket for the Server.

For more details, please refer to Kerberos article.

47.  What is HSRP?

Hot Standby Router Protocol (HSRP) is a CISCO proprietary protocol, which provides redundancy for a local subnet. In HSRP, two or more routers give an illusion of a virtual router.

HSRP allows you to configure two or more routers as standby routers and only a single router as an active router at a time. All the routers in a single HSRP group share a single MAC address and IP address, which acts as a default gateway to the local network. The Active router is responsible for forwarding the traffic. If it fails, the Standby router takes up all the responsibilities of the active router and forwards the traffic.

For more details, please refer to HSRP protocol.

48. Why is the MAC address called the Physical address?

The MAC address is a physical address (also called a hardware address) because it physically identifies an item of hardware. MAC addresses use three types of number systems and all use the same format, only the size of the identifier differs. The addresses can be “Universally Managed” or “Locally Managed”. For more details, please refer to Introduction of MAC Address in Computer Network article.

49. Process of DHCP(DORA)?

In DHCP, the client and the server exchange mainly 4 DHCP messages in order to make a connection. This process is known as  DORA process (discovery, offer, request, and acknowledgment), but there are 8 DHCP messages in the process.  For more details, please refer to Dynamic Host Configuration Protocol (DHCP) article.

50.  What is ‘APIPA’?

APIPA stands for Automatic Private IP Addressing (APIPA). It is a feature or characteristic in operating systems (eg. Windows) which enables computers to self-configure an IP address and subnet mask automatically when their DHCP(Dynamic Host Configuration Protocol) server isn’t reachable. The IP address range for APIPA is (169.254.0.1 to 169.254.255.254) having 65, 534 usable IP addresses, with the subnet mask of 255.255.0.0.

For more details please read  What is APIPA article.