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Networking is a cornerstone of system design. This guide covers the fundamental networking concepts you’ll encounter in interviews.

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OSI Model

OSI Model How is data sent over the network? Why do we need so many layers in the OSI model? The diagram shows how data is encapsulated and de-encapsulated when transmitting over the network.

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Data Encapsulation Process

1

Application Layer

When Device A sends data to Device B over the network via the HTTP protocol, it is first added an HTTP header at the application layer.
2

Transport Layer

Then a TCP or a UDP header is added to the data. It is encapsulated into TCP segments at the transport layer. The header contains the source port, destination port, and sequence number.
3

Network Layer

The segments are then encapsulated with an IP header at the network layer. The IP header contains the source/destination IP addresses.
4

Data Link Layer

The IP datagram is added a MAC header at the data link layer, with source/destination MAC addresses.
5

Physical Layer

The encapsulated frames are sent to the physical layer and sent over the network in binary bits.
6

De-encapsulation

When Device B receives the bits from the network, it performs the de-encapsulation process, which is a reverse processing of the encapsulation process. The headers are removed layer by layer, and eventually, Device B can read the data.
We need layers in the network model because each layer focuses on its own responsibilities. Each layer can rely on the headers for processing instructions and does not need to know the meaning of the data from the last layer.

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Network Protocols

Network Protocols Network protocols are standard methods of transferring data between two computers in a network.

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Common Protocols

HTTP is a protocol for fetching resources such as HTML documents. It is the foundation of any data exchange on the Web and it is a client-server protocol.Key Features:
  • Request-response model
  • Stateless protocol
  • Port 80 (default)
  • Built on TCP
HTTP/3 is the next major revision of the HTTP. It runs on QUIC, a new transport protocol designed for mobile-heavy internet usage.Key Features:
  • Built on UDP instead of TCP
  • Faster web page responsiveness
  • Better for VR applications
  • Reduced latency
HTTPS extends HTTP and uses encryption for secure communications.Key Features:
  • TLS/SSL encryption
  • Port 443 (default)
  • Certificate validation
  • Secure data transmission
WebSocket is a protocol that provides full-duplex communications over TCP.Key Features:
  • Bidirectional communication
  • Real-time updates
  • Persistent connection
  • Used in gaming, trading, messaging
Unlike REST, which always “pulls” data, WebSocket enables data to be “pushed”.
TCP is designed to send packets across the internet and ensure the successful delivery of data and messages over networks.Key Features:
  • Connection-oriented
  • Reliable delivery
  • Ordered packets
  • Error checking
Many application-layer protocols build on top of TCP.
UDP sends packets directly to a target computer, without establishing a connection first.Key Features:
  • Connectionless
  • No delivery guarantee
  • Lower latency
  • Lightweight
Voice and video traffic are often sent using this protocol.
SMTP is a standard protocol to transfer electronic mail from one user to another.Key Features:
  • Port 25 (default)
  • Email transmission
  • Text-based protocol
  • Push protocol
FTP is used to transfer computer files between client and server.Key Features:
  • Port 21 (control)
  • Separate data channel
  • Authentication required
  • Bidirectional transfer

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TCP vs UDP

TCP Use Cases

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When Reliability Matters

Best for:
  • Web browsing (HTTP/HTTPS)
  • Email (SMTP, IMAP, POP3)
  • File transfers (FTP, SFTP)
  • Database queries
  • APIs (REST, GraphQL)
Why TCP:
  • Guaranteed delivery
  • Order preservation
  • Error correction
  • Flow control

UDP Use Cases

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When Speed Matters

Best for:
  • Live video streaming
  • VoIP (Voice over IP)
  • DNS queries
  • Market data multicast
  • IoT communications
  • Online gaming
Why UDP:
  • Lower latency
  • No connection overhead
  • Tolerates packet loss
  • Lightweight
UDP Use Cases

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DNS (Domain Name System)

DNS Lookup DNS acts as an address book. It translates human-readable domain names (google.com) to machine-readable IP addresses (142.251.46.238).

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DNS Hierarchy

To achieve better scalability, the DNS servers are organized in a hierarchical tree structure.
Root Name Server (.)
  • Stores IP addresses of TLD name servers
  • 13 logical root name servers globally
  • First point of contact in DNS resolution
  • Operated by different organizations

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DNS Lookup Process

1

Browser Query

google.com is typed into the browser, and the browser sends the domain name to the DNS resolver.
2

Root Server Query

The resolver queries a DNS root name server.
3

Root Server Response

The root server responds to the resolver with the address of a TLD DNS server. In this case, it is .com.
4

TLD Query

The resolver then makes a request to the .com TLD.
5

TLD Response

The TLD server responds with the IP address of the domain’s name server, google.com (authoritative name server).
6

Authoritative Query

The DNS resolver sends a query to the domain’s nameserver.
7

IP Address Return

The IP address for google.com is then returned to the resolver from the nameserver.
8

Browser Response

The DNS resolver responds to the web browser with the IP address (142.251.46.238) of the domain requested initially.
DNS lookups on average take between 20-120 milliseconds to complete (according to YSlow).

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DNS Record Types

DNS Record Types

A Record

Address RecordMaps a domain name to an IPv4 address. One of the most essential records for translating human-readable domain names into IP addresses.

AAAA Record

IPv6 Address RecordSimilar to an A record but maps a domain name to an IPv6 address. Used for websites and services that support the IPv6 protocol.

CNAME Record

Canonical Name RecordUsed to alias one domain name to another. Often used for subdomains, pointing them to the main domain while keeping the actual domain name hidden.

MX Record

Mail Exchange RecordDirects email traffic to the correct mail server. Essential for email routing.

NS Record

Name Server RecordSpecifies the authoritative DNS servers for the domain. These records help direct queries to the correct DNS servers for further lookups.

PTR Record

Pointer RecordProvides reverse DNS lookup, mapping an IP address back to a domain name. Commonly used in verifying the authenticity of a server.

SRV Record

Service RecordSpecifies a host and port for specific services such as VoIP. Used in conjunction with A records.

TXT Record

Text RecordAllows administrators to add human-readable text to DNS records. Used to include verification records, like SPF, for email security.

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IP Addressing

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IPv4 vs IPv6

IPv4 vs IPv6 The transition from IPv4 to IPv6 is primarily driven by the need for more internet addresses, alongside the desire to streamline certain aspects of network management.
Format and Length
  • Address Size: 32-bit
  • Format: Four decimal numbers separated by dots
  • Example: 192.168.0.12
  • Total Addresses: ~4.3 billion
  • Status: Address exhaustion
Header ComplexityThe IPv4 header is more complex and includes fields such as:
  • Header length
  • Service type
  • Total length
  • Identification, flags, fragment offset
  • Time to live (TTL)
  • Protocol
  • Header checksum
  • Source and destination IP addresses
  • Options

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Network Types

Network Types Different types of networks are defined based on their size, range, and purpose.
A PAN is a network used for communication among devices close to one person, typically within a range of a few meters.Use Cases:
  • Connecting personal devices like smartphones, tablets, and wearables
  • Enabling hands-free communication through Bluetooth headsets
  • Synchronizing data between a computer and a smartphone
Range: ~10 meters
A LAN is a network that connects computers and devices within a limited area such as a home, office, or building.Use Cases:
  • Sharing resources like printers and file servers within an office
  • Facilitating communication and collaboration among employees
  • Providing internet access within a home or small business
Range: ~100-1000 meters
A MAN covers a larger geographic area than a LAN but smaller than a WAN, typically spanning a city or a large campus.Use Cases:
  • Connecting multiple campuses of a university
  • Providing high-speed internet access across a city
  • Linking local government offices within a metropolitan area
Range: ~10 kilometers
A WAN spans a large geographic area, often a country or continent. The most prominent example of a WAN is the Internet.Use Cases:
  • Connecting branch offices of multinational companies
  • Facilitating global communication and data exchange
  • Enabling remote access to central resources
Range: Unlimited (global)

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HTTP Evolution

HTTP Evolution
HTTP/1 (1996) and HTTP/1.1 (1997)
  • Persistent connections
  • Pipelining
  • Header support
  • Built on TCP
  • Reliable communication
  • Still widely used (25+ years old)
Limitations:
  • Head-of-line blocking
  • No multiplexing
  • Redundant headers
  • Sequential requests

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HTTPS and Security

HTTPS HTTPS is an extension of HTTP that transmits encrypted data using Transport Layer Security (TLS). If the data is hijacked online, all the hijacker gets is binary code.

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How HTTPS Works

1

TCP Connection

The client (browser) and the server establish a TCP connection.
2

TLS Handshake

The client sends a “client hello” to the server. The message contains a set of necessary encryption algorithms (cipher suites) and the latest TLS version it can support. The server responds with a “server hello” so the browser knows whether it can support the algorithms and TLS version.The server then sends the SSL certificate to the client. The certificate contains the public key, hostname, expiry dates, etc. The client validates the certificate.
3

Session Key Exchange

After validating the SSL certificate, the client generates a session key and encrypts it using the public key. The server receives the encrypted session key and decrypts it with the private key.
4

Secure Communication

Now that both the client and the server hold the same session key (symmetric encryption), the encrypted data is transmitted in a secure bi-directional channel.

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Why Switch to Symmetric Encryption?

Security

The asymmetric encryption goes only one way. This means that if the server tries to send the encrypted data back to the client, anyone can decrypt the data using the public key.

Performance

The asymmetric encryption adds quite a lot of mathematical overhead. It is not suitable for data transmissions in long sessions.

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Common Ports

Common Ports
PortProtocolDescription
21FTPFile Transfer Protocol
22SSHSecure Shell for Login
23TelnetRemote login (insecure)
25SMTPSimple Mail Transfer Protocol
80HTTPHypertext Transfer Protocol
110POP3Post Office Protocol V3
143IMAPInternet Message Access Protocol
443HTTPSSecure HTTP

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URL Components

URL Structure Uniform Resource Locator (URL) is used to locate resources on the internet. URLs comprise several components:
The protocol or scheme, such as http, https, ftp, wsExample: https://
The domain name and port, separated by a period (.)Example: www.example.com:443
  • Domain: www.example.com
  • Port: 443 (optional, defaults based on protocol)
The path to the resource, separated by a slash (/)Example: /products/category/item
The parameters, which start with a question mark (?) and consist of key-value pairsExample: ?id=123&color=blue&size=largeFormat: key1=value1&key2=value2
Indicated by a pound sign (#), used to bookmark a specific section of the resourceExample: #section-headerNote: Not sent to the server, processed by browser only

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Interview Tips

Common Mistakes
  • Confusing HTTP/2 and HTTP/3 transport protocols (TCP vs UDP)
  • Not understanding the difference between TCP and UDP trade-offs
  • Forgetting that DNS has caching at multiple levels
  • Mixing up OSI model layers and their responsibilities
Interview Strategy: When discussing networking in system design, always consider:
  • Protocol choice (TCP vs UDP)
  • DNS resolution and caching
  • Network latency and bandwidth
  • Security requirements (HTTPS, TLS)
  • Geographic distribution (CDN, edge networks)

Protocol Selection

Know when to choose TCP (reliability) vs UDP (speed) based on application requirements.

DNS Optimization

Understand DNS caching strategies and TTL values to reduce latency.

Security First

Always consider HTTPS, TLS versions, and certificate management in production systems.

Network Types

Know the appropriate network type (LAN, WAN, etc.) for different system components.

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