Top 20 Computer Networks Interview Questions

The OSI (Open Systems Interconnection) model is a conceptual framework used to standardize how network protocols communicate. The seven layers, from bottom to top, are: Physical, Data Link, Network, Transport, Session, Presentation, and Application.

TCP (Transmission Control Protocol) is a reliable, connection-oriented protocol that guarantees data delivery and correct ordering (used for web browsing, emails). UDP (User Datagram Protocol) is a connectionless, fast protocol that does not guarantee delivery or ordering (used for live streaming, online gaming).

It is the process used by TCP to establish a reliable connection between a client and a server before any data is sent. The client sends a SYN (synchronize) packet, the server replies with a SYN-ACK (synchronize-acknowledge) packet, and the client finishes by sending an ACK (acknowledge) packet.

DNS is the phonebook of the internet. It translates human-readable domain names (like google.com) into numerical IP addresses (like 142.250.190.46) that computers use to locate each other on a network.

DHCP is a network management protocol that automatically assigns an IP address and other configuration settings to devices when they join a network. This saves administrators from having to manually configure every single device.

An IP address is a logical address assigned to a device on a network, which changes depending on where the device connects. A MAC address is a permanent physical address hardcoded into a device's Network Interface Card (NIC) by the manufacturer.

IPv4 uses a 32-bit address format (allowing ~4.3 billion unique addresses) written in decimals. IPv6 was created to solve IPv4 exhaustion; it uses a 128-bit address format (providing virtually unlimited addresses) written in hexadecimal.

A public IP address is globally unique and assigned by your ISP to connect your router directly to the internet. A private IP address is used internally within a local network (like your home Wi-Fi) and cannot be routed directly on the public internet.

NAT is a process performed by a router that translates multiple private IP addresses on a local network into a single public IP address before sending traffic out to the internet. It conserves IPv4 addresses and hides internal devices from the outside world.

Subnetting is the practice of dividing a single large network into smaller, logical sub-networks (subnets). It improves network performance, reduces broadcast traffic, and enhances security by isolating groups of devices.

ARP is a protocol used to map an IP address to a MAC address on a local network. When a device needs to communicate with another device on the same network but only knows its IP, it broadcasts an ARP request to find the matching MAC address.

A Hub is a basic Layer 1 device that blindly broadcasts incoming data to all connected devices. A Switch is a smarter Layer 2 device that sends data only to the specific intended device using MAC addresses. A Router is a Layer 3 device that connects completely different networks together using IP addresses.

HTTP (HyperText Transfer Protocol) transmits web data in plain text, making it vulnerable to interception. HTTPS adds an encryption layer (using SSL/TLS) so that data exchanged between the browser and the server is completely secure.

A firewall is a network security system (hardware or software) that monitors and controls incoming and outgoing network traffic. It acts as a barrier between a trusted internal network and untrusted external networks based on predefined security rules.

A VPN creates a secure, encrypted tunnel between a user's device and the internet over a public network. It hides the user's real IP address and encrypts data to provide privacy and bypass geographical restrictions.

LAN (Local Area Network) covers a small, localized area like a single home or office building. MAN (Metropolitan Area Network) spans a larger area, such as a college campus or a city. WAN (Wide Area Network) covers massive geographical areas, like a country or the entire globe (the Internet is a WAN).

First, the browser checks its cache for the IP address. If missing, it queries a DNS server to resolve the domain name. The browser then initiates a TCP 3-way handshake with the server, sends an HTTP/HTTPS request, receives the server's response, and renders the web page.

ICMP (Internet Control Message Protocol) is used by network devices to send error messages and operational information. The `ping` command uses ICMP Echo Request and Echo Reply messages to test if a specific IP address is reachable and to measure the round-trip time.

Unicast is one-to-one communication between a single sender and a single receiver. Multicast is one-to-many communication directed only to a specific, subscribed group of receivers. Broadcast is one-to-all communication sent to every device on that local network segment.

A default gateway is a network node (almost always a router) that serves as the exit point to other networks. When a device wants to send data to an IP address outside its own local subnet, it forwards that data to the default gateway to be routed correctly.