🔹 Q1: What is an IP address?

A: An IP (Internet Protocol) address is a unique identifier assigned to each device connected to a network. It helps in locating and identifying the device on the internet or local network.

🔹 Q2: Difference between public and private IP address?

A: - Public IP: Globally unique, accessible over the internet.

Private IP: Used within a local network; not routable on the internet (e.g., 192.168.x.x, 10.x.x.x).

🔹 Q3: What is a port?

A: A port is a logical access channel for communication between two devices. Each service on a device runs on a specific port (e.g., HTTP – port 80, SSH – port 22).

🔹 Q4: Difference between TCP and UDP?

TCP (Transmission Control Protocol): Connection-oriented, reliable, slower (e.g., HTTP, SSH).
UDP (User Datagram Protocol): Connectionless, faster, but not reliable (e.g., DNS, streaming).

🔹 Q5: What is DNS?

A: DNS (Domain Name System) translates human-readable domain names (like google.com) into IP addresses (like 142.250.190.78).

🔹 Q6: What is a socket?

A: A socket is a combination of an IP address and a port number, used to establish a communication endpoint between two devices.

🔹 Q7: What is NAT (Network Address Translation)?

A: NAT translates private IP addresses to public IP addresses and vice versa, allowing multiple devices in a private network to access the internet using one public IP.

🔹 Q8: What is a subnet?

A: A subnet is a segmented piece of a larger network, used to divide and manage networks efficiently. It’s defined by a subnet mask or CIDR notation.

🔹 Q9: What is CIDR?

A: CIDR (Classless Inter-Domain Routing) is a method for allocating IP addresses and routing. Example: 192.168.1.0/24 means 256 IPs (from .0 to .255).

🔹 Q10: What is the difference between HTTP and HTTPS?

HTTP: Unsecured communication over port 80.
HTTPS: Secure version using SSL/TLS encryption, over port 443.

🔹 Q11: What is a firewall?

A: A firewall is a security system that monitors and controls incoming and outgoing traffic based on predefined rules.

🔹 Q12: What is the difference between Load Balancer and Reverse Proxy?

Load Balancer: Distributes incoming traffic across multiple servers.
Reverse Proxy: Forwards client requests to backend servers and returns the response. Nginx can act as both.

🔹 Q13: Common Networking Commands?

ping <host> – Test if host is reachable.
traceroute <host> – Show path to host.
nslookup <domain> – Check DNS resolution.
netstat -tulnp – List active ports.
curl <url> – Make web requests.
telnet <host> <port> – Test connectivity to port.

🔹 Q14: What happens when you type a URL in the browser?

DNS lookup to resolve domain name.
TCP handshake between client and server.
HTTP/HTTPS request sent.
Server responds with data.
Browser renders the content.

🔹 Q15: What is a VPC?

A: VPC (Virtual Private Cloud) is a logically isolated network in the cloud (like AWS) where you can launch resources (EC2, databases) in private or public subnets.

🔹 Q16: What is the difference between Ingress and Egress traffic?

Ingress: Incoming traffic to a system/network.
Egress: Outgoing traffic from a system/network.

🔹 Q17: What is a CDN?

A: CDN (Content Delivery Network) caches and delivers content from servers located close to users, improving load time and availability.

🔹 Q18: What are some common ports?

HTTP – 80
HTTPS – 443
SSH – 22
FTP – 21
DNS – 53
MySQL – 3306
PostgreSQL – 5432
Kubernetes API – 6443

🔹 Q19: What is MTU?

A: MTU (Maximum Transmission Unit) is the largest size of a packet that can be sent in a network layer protocol. Mismatched MTUs can cause packet fragmentation or drops.

🔹 Q20: How do microservices communicate with each other in Kubernetes?

A: Usually over internal DNS names and cluster IPs. For example, http://service-name.namespace.svc.cluster.local.

🔹 Q21: What is a MAC address?

A: A MAC (Media Access Control) address is a hardware identifier assigned to a network interface card (NIC) used for communication within the local network.

🔹 Q22: What is ARP?

A: ARP (Address Resolution Protocol) maps IP addresses to MAC addresses so that devices can communicate within a local network.

🔹 Q23: What is the difference between Layer 2 and Layer 3?

Layer 2 (Data Link Layer): Works with MAC addresses (e.g., switches).
Layer 3 (Network Layer): Works with IP addresses (e.g., routers).

🔹 Q24: What is a routing table?

A: A routing table is a data table stored in a router that lists the paths to particular network destinations.

🔹 Q25: What is a default gateway?

A: The default gateway is the device (usually a router) that routes traffic from a local network to external networks or the internet.

🔹 Q26: What is a DHCP server?

A: DHCP (Dynamic Host Configuration Protocol) server dynamically assigns IP addresses and network configuration to client devices.

🔹 Q27: What is the difference between Static IP and Dynamic IP?

Static IP: Manually assigned and fixed.
Dynamic IP: Automatically assigned by a DHCP server.

🔹 Q28: What is port forwarding?

A: Port forwarding maps an external port to an internal IP and port, allowing access to services inside a private network from the internet.

🔹 Q29: What is a proxy server?

A: A proxy server acts as an intermediary between client and server, used for caching, filtering, logging, and hiding IP addresses.

🔹 Q30: What is a reverse proxy?

A: A reverse proxy (like Nginx or HAProxy) routes client requests to the appropriate backend server and returns the response to the client.

🔹 Q31: What is the difference between Load Balancer and Auto Scaling?

Load Balancer: Distributes traffic to healthy instances
Auto Scaling: Dynamically increases/decreases the number of instances based on traffic/load.

🔹 Q32: How do containers communicate in Docker?

A: By default, Docker containers in the same bridge network can communicate via container name or IP. You can also use custom networks or expose ports to host.

🔹 Q33: What is `iptables`?

A: iptables is a Linux utility used to configure the firewall rules for packet filtering and NAT.

🔹 Q34: What is a service mesh?

A: A service mesh (e.g., Istio, Linkerd) is a dedicated infrastructure layer to control service-to-service communication in a microservices architecture.

🔹 Q35: What is a network namespace?

A: Network namespaces provide isolated network stacks (interfaces, routing tables, etc.) for processes — used heavily in containers and Kubernetes pods.

🔹 Q36: What is the difference between SNAT and DNAT?

SNAT (Source NAT): Changes source IP (used in outbound traffic).
DNAT (Destination NAT): Changes destination IP (used in inbound traffic).

🔹 Q37: What is a health check in a load balancer?

A: It’s a mechanism to determine if a backend server is healthy and able to serve traffic. Unhealthy servers are removed from the load balancing pool.

🔹 Q38: What is connection pooling?

A: It reuses network connections instead of opening a new one for every request, reducing overhead and improving performance.

🔹 Q39: How can you test if a port is open on a remote server?

Use telnet <host> <port>
Or nc -zv <host> <port>

🔹 Q40: What is a VPN?

A: A VPN (Virtual Private Network) encrypts your internet connection and routes it through a secure server, used to securely connect remote systems to a private network.

Absolutely! The OSI (Open Systems Interconnection) Model is a foundational networking concept — and a must-know for DevOps interviews, especially when asked about protocols, ports, firewalls, load balancers, etc.

📶 OSI Model – Explained with Interview Q&A

The OSI model has 7 layers, each with a specific role in data communication.

🌐 OSI Model – Layers (Top to Bottom):

Layer No.	Layer Name	Function Summary	Examples
7	Application	User interface; network services	HTTP, HTTPS, FTP, SSH, DNS
6	Presentation	Data formatting, encryption, compression	SSL/TLS, JPEG, MP3
5	Session	Session management between apps	API calls, NetBIOS
4	Transport	Reliable delivery, error handling	TCP, UDP
3	Network	Routing, addressing	IP, ICMP, Routers
2	Data Link	MAC addressing, frame delivery	Ethernet, MAC, Switches
1	Physical	Transmission over physical media	Cables, Hubs, Signals, Bits

🔹 Q1: What is the OSI model?

A: The OSI model is a conceptual framework that standardizes the functions of a telecommunication or computing system into 7 distinct layers, enabling interoperability between different systems and protocols.

🔹 Q2: Which layer does IP work on?

A: IP works on the Network Layer (Layer 3).

🔹 Q3: Which protocols work at the Transport layer?

A: TCP and UDP are the main protocols at the Transport Layer (Layer 4).

🔹 Q4: What is the role of the Application Layer?

A: It provides services directly to the user or application, like file transfers, emails, or browser data. Examples: HTTP, FTP, DNS.

🔹 Q5: What is the function of the Presentation Layer?

A: It handles encryption, decryption, data translation, and compression. For example, converting text to ASCII or encrypting via TLS.

🔹 Q6: What layer does a router operate at?

A: Network Layer (Layer 3).

🔹 Q7: What layer does a switch operate at?

A: Traditional switches operate at the Data Link Layer (Layer 2). Layer 3 switches can route as well.

🔹 Q8: What is the difference between TCP and UDP at the OSI level?

TCP (Layer 4): Connection-oriented, reliable, ensures data arrives in order.
UDP (Layer 4): Connectionless, faster but no guarantee of delivery.

🔹 Q9: At which layer does SSL/TLS work?

A: Mostly in the Presentation Layer (Layer 6) but often associated with Application Layer in practice (Layer 7).

🔹 Q10: What is encapsulation in the OSI model?

A: Encapsulation is the process of wrapping data with protocol-specific information at each layer (headers/footers), from Application to Physical.

📦 DevOps Relevance of OSI Layers

DevOps Tool/Topic	OSI Layer(s) Involved
HTTP/HTTPS, APIs	Layer 7 – Application
TLS/SSL (e.g., HTTPS)	Layer 6 – Presentation
TCP, UDP Port Configs	Layer 4 – Transport
IP routing, VPC setup	Layer 3 – Network
MAC filtering, VLANs	Layer 2 – Data Link
Physical network setup	Layer 1 – Physical

✅ Pro Tip for Interviews:

Always give examples from tools you’ve used, like:

“For example, when I exposed a Docker container, I had to configure the port (Layer 4), IP (Layer 3), and ensure the web app over HTTP was reachable (Layer 7).”

Notes

Explorer

Computer Networking