Networking on Software Engineering Learning Paths

OSI and TCP/IP Models

Mon, 01 Jan 0001 00:00:00 +0000

Why Layered Models?#

Networking is complex — it involves physical signals, addressing, routing, reliability, and application logic. Layered models break this complexity into manageable pieces where each layer has a single responsibility and communicates only with adjacent layers.

flowchart TD
 App["Application: What to send"] --> Transport["Transport: How to deliver reliably"]
 Transport --> Network["Network: Where to send (routing)"]
 Network --> DataLink["Data Link: Local delivery (MAC)"]
 DataLink --> Physical["Physical: Bits on the wire"]

The OSI Model (7 Layers)#

A theoretical reference model — useful for understanding and troubleshooting:

Physical and Data Link Layers

Mon, 01 Jan 0001 00:00:00 +0000

Physical Layer (Layer 1)#

The physical layer converts bits into signals and transmits them over a medium.

Transmission Media#

Medium	Speed	Distance	Use Case
Cat5e (copper)	1 Gbps	100m	Office LAN
Cat6a (copper)	10 Gbps	100m	Data center
Single-mode fiber	100+ Gbps	80+ km	Long-haul, WAN
Multi-mode fiber	10-100 Gbps	300-550m	Data center
Wi-Fi 6 (802.11ax)	9.6 Gbps (theoretical)	~30m indoor	Wireless LAN

Data Link Layer (Layer 2)#

Responsible for local delivery — getting frames from one device to another on the same network segment.

IP Addressing

Mon, 01 Jan 0001 00:00:00 +0000

IPv4 Addresses#

A 32-bit number that uniquely identifies a device on a network, written as four decimal octets:

192.168.1.100
 │ │ │ │
 └────┴───┴──┴── Each octet: 0-255 (8 bits)

Binary: 11000000.10101000.00000001.01100100
Decimal: 192 . 168 . 1 . 100

Subnetting and CIDR#

CIDR Notation#

CIDR (Classless Inter-Domain Routing) notation: IP/prefix_length

The prefix identifies the network portion, the remaining bits identify the host:

10.0.1.50/24
├── Network: 10.0.1 (first 24 bits — fixed)
└── Host: .50 (last 8 bits — variable)

Network address: 10.0.1.0 (all host bits = 0)
Broadcast: 10.0.1.255 (all host bits = 1)
Usable hosts: 10.0.1.1 – 10.0.1.254 (254 hosts)

Subnet Mask#

The subnet mask marks which bits are network (1) and which are host (0):

Routing

Mon, 01 Jan 0001 00:00:00 +0000

What is Routing?#

Routing is the process of selecting a path for network traffic across one or more networks. Routers examine the destination IP address of each packet and decide where to forward it next.

flowchart LR
 A["Source<br/>192.168.1.10"] --> R1["Router 1"]
 R1 --> R2["Router 2"]
 R2 --> R3["Router 3"]
 R3 --> B["Destination<br/>203.0.113.50"]

Each router makes an independent forwarding decision based on its routing table — it doesn’t know the full path, only the next hop.

Transport Layer: TCP vs UDP

Mon, 01 Jan 0001 00:00:00 +0000

Role of the Transport Layer#

The transport layer provides end-to-end communication between applications on different hosts. It multiplexes connections using ports and (in TCP’s case) ensures reliable delivery.

flowchart TD
 App1["Web Browser (port 443)"] --> TL["Transport Layer"]
 App2["SSH Client (port 22)"] --> TL
 App3["DNS Client (port 53)"] --> TL
 TL --> NL["Network Layer (IP)"]

TCP (Transmission Control Protocol)#

Reliable, ordered, connection-oriented. Used when every byte must arrive correctly.

Three-Way Handshake#

sequenceDiagram
 participant C as Client
 participant S as Server
 
 C->>S: SYN (seq=100)
 S->>C: SYN-ACK (seq=300, ack=101)
 C->>S: ACK (seq=101, ack=301)
 Note over C,S: Connection established

TCP Features#

Feature	Mechanism
Reliability	Acknowledgments + retransmission
Ordering	Sequence numbers
Flow control	Sliding window (receiver advertises capacity)
Congestion control	Slow start, congestion avoidance, fast retransmit
Connection state	SYN → ESTABLISHED → FIN → CLOSED

TCP Connection Teardown#

sequenceDiagram
 participant C as Client
 participant S as Server
 
 C->>S: FIN (I'm done sending)
 S->>C: ACK
 S->>C: FIN (I'm done too)
 C->>S: ACK
 Note over C: TIME_WAIT (2×MSL)

TIME_WAIT: Client waits ~60s to ensure the final ACK arrives. This is why you see many TIME_WAIT connections on busy servers.

DNS (Domain Name System)

Mon, 01 Jan 0001 00:00:00 +0000

What is DNS?#

DNS translates human-readable domain names into IP addresses. It’s a distributed, hierarchical database — the “phone book” of the internet.

flowchart LR
 Browser["www.example.com"] -->|DNS Query| Resolver["DNS Resolver"]
 Resolver -->|"93.184.216.34"| Browser

Without DNS, you’d need to memorize IP addresses for every website.

DNS Hierarchy#

flowchart TD
 Root[". (Root)"]
 Root --> COM[".com"]
 Root --> ORG[".org"]
 Root --> NET[".net"]
 COM --> Example["example.com"]
 COM --> Google["google.com"]
 Example --> WWW["www.example.com"]
 Example --> API["api.example.com"]

Level	Example	Managed By
Root	`.`	ICANN (13 root server clusters)
TLD	`.com`, `.org`, `.io`	Registry operators
Domain	`example.com`	Domain owner
Subdomain	`www.example.com`	Domain owner

DNS Resolution Process#

sequenceDiagram
 participant B as Browser
 participant R as Recursive Resolver (ISP/8.8.8.8)
 participant Root as Root Server
 participant TLD as .com TLD Server
 participant Auth as example.com Authoritative

 B->>R: What is www.example.com?
 R->>Root: What is www.example.com?
 Root->>R: I don't know, ask .com TLD (ns1.verisign.com)
 R->>TLD: What is www.example.com?
 TLD->>R: Ask example.com nameservers (ns1.example.com)
 R->>Auth: What is www.example.com?
 Auth->>R: A record: 93.184.216.34 (TTL: 3600)
 R->>B: 93.184.216.34 (cached for 1 hour)

Resolver Types#

Type	Role	Example
Stub resolver	Client’s local DNS (asks recursive)	Your OS
Recursive resolver	Does the full lookup	8.8.8.8, 1.1.1.1, ISP DNS
Authoritative	Holds the actual records	ns1.example.com

DNS Record Types#

Type	Purpose	Example
A	IPv4 address	`example.com → 93.184.216.34`
AAAA	IPv6 address	`example.com → 2606:2800:220:1:...`
CNAME	Alias to another name	`www.example.com → example.com`
MX	Mail server	`example.com → mail.example.com (pri 10)`
TXT	Arbitrary text	SPF, DKIM, domain verification
NS	Nameserver delegation	`example.com → ns1.example.com`
SOA	Zone authority metadata	Serial, refresh, retry, expire
SRV	Service location	`_http._tcp.example.com → web:8080`
PTR	Reverse DNS (IP → name)	`34.216.184.93 → example.com`
CAA	Certificate authority authorization	Which CAs can issue certs

CNAME Rules#

Cannot coexist with other records at the same name
Cannot be used at the zone apex (example.com) — only subdomains
AWS Route 53 “Alias” records solve the apex limitation

TTL (Time To Live)#

How long resolvers cache a record (in seconds):

HTTP/HTTPS

Mon, 01 Jan 0001 00:00:00 +0000

What is HTTP?#

HTTP (HyperText Transfer Protocol) is the application-layer protocol for the web. It’s a request-response protocol — a client sends a request, a server returns a response.

sequenceDiagram
 participant C as Client
 participant S as Server
 
 C->>S: GET /api/users HTTP/1.1
 S->>C: HTTP/1.1 200 OK + body

HTTP Request Structure#

GET /api/users?page=1 HTTP/1.1 ← Request line
Host: api.example.com ← Headers
Authorization: Bearer eyJhbG...
Accept: application/json
 ← Empty line (end of headers)
 ← Body (for POST/PUT)

HTTP Methods#

Method	Purpose	Idempotent	Safe	Has Body
GET	Retrieve resource	✅	✅	No
POST	Create resource	❌	❌	Yes
PUT	Replace resource entirely	✅	❌	Yes
PATCH	Partial update	❌	❌	Yes
DELETE	Remove resource	✅	❌	Optional
HEAD	Get headers only (no body)	✅	✅	No
OPTIONS	Get allowed methods (CORS preflight)	✅	✅	No

Idempotent: Multiple identical requests have the same effect as one. Safe: Doesn’t modify server state.

Network Security

Mon, 01 Jan 0001 00:00:00 +0000

Defense in Depth#

Network security uses multiple layers — if one fails, others still protect:

flowchart TD
 Internet["Internet"]
 Internet --> Edge["Edge: WAF, DDoS protection"]
 Edge --> FW["Perimeter: Firewall"]
 FW --> SG["Host: Security Groups / iptables"]
 SG --> App["Application: Auth, input validation"]
 App --> Data["Data: Encryption at rest"]

Firewalls#

Types#

Type	Layer	What It Inspects	Example
Packet filter	3-4	IP, port, protocol	iptables, ACLs
Stateful	3-4	Connection state + packets	AWS Security Groups
Application (WAF)	7	HTTP content, patterns	AWS WAF, Cloudflare
Next-gen (NGFW)	3-7	Deep packet inspection	Palo Alto, Fortinet

Stateful vs Stateless#

Aspect	Stateless (NACLs)	Stateful (Security Groups)
Tracks connections	No	Yes
Return traffic	Must explicitly allow	Automatically allowed
Rule evaluation	All rules, in order	All rules, most permissive wins
Use case	Subnet-level control	Instance-level control

Security Group Example (AWS)#

Inbound Rules:
 Allow TCP 443 from 0.0.0.0/0 (HTTPS from anywhere)
 Allow TCP 22 from 10.0.0.0/16 (SSH from VPC only)

Outbound Rules:
 Allow All traffic to 0.0.0.0/0 (default — allow all outbound)

Principle of least privilege: Only open ports that are needed, only from sources that need access.

Load Balancing and Reverse Proxies

Mon, 01 Jan 0001 00:00:00 +0000

Why Load Balance?#

A single server has finite capacity. Load balancing distributes traffic across multiple servers to achieve:

High availability — if one server fails, others handle traffic
Scalability — add servers to handle more load
Performance — distribute work evenly

flowchart LR
 C1["Client"] --> LB["Load Balancer"]
 C2["Client"] --> LB
 C3["Client"] --> LB
 
 LB --> S1["Server 1"]
 LB --> S2["Server 2"]
 LB --> S3["Server 3"]

Load Balancing Algorithms#

Algorithm	How It Works	Best For
Round Robin	Rotate sequentially	Equal-capacity servers
Weighted Round Robin	Rotate with weights (2:1:1)	Mixed-capacity servers
Least Connections	Send to server with fewest active connections	Variable request duration
Least Response Time	Send to fastest-responding server	Heterogeneous backends
IP Hash	Hash client IP → consistent server	Session affinity without cookies
Random	Random selection	Simple, effective at scale

Choosing an Algorithm#

flowchart TD
 Q1{"All servers equal capacity?"}
 Q1 -->|Yes| Q2{"Requests take similar time?"}
 Q1 -->|No| WRR["Weighted Round Robin"]
 Q2 -->|Yes| RR["Round Robin"]
 Q2 -->|No| LC["Least Connections"]

Layer 4 vs Layer 7 Load Balancing#

Aspect	Layer 4 (Transport)	Layer 7 (Application)
Inspects	IP + Port	HTTP headers, URL, cookies
Speed	Faster (less processing)	Slower (deep inspection)
Routing decisions	Based on IP/port only	Based on URL path, headers, content
TLS termination	Pass-through or terminate	Always terminates
Use case	TCP/UDP services, databases	Web applications, APIs

Layer 7 Routing Examples#

/api/* → API server pool
/static/* → CDN or static server pool
/admin/* → Admin server (restricted)
Host: mobile.* → Mobile-optimized servers

AWS Load Balancer Types#

Type	Layer	Protocols	Use Case
ALB (Application)	7	HTTP, HTTPS, gRPC	Web apps, APIs, microservices
NLB (Network)	4	TCP, UDP, TLS	High performance, static IPs
CLB (Classic)	4/7	HTTP, TCP	Legacy (avoid for new projects)

Health Checks#

Load balancers must know which backends are healthy:

Troubleshooting Tools

Mon, 01 Jan 0001 00:00:00 +0000

Systematic Approach#

Network troubleshooting follows the layers — start at the bottom and work up:

flowchart TD
 A["Can't reach service"] --> B{"Can ping IP?"}
 B -->|No| C{"Can ping gateway?"}
 C -->|No| D["Layer 1-2: Cable, NIC, switch"]
 C -->|Yes| E["Layer 3: Routing issue"]
 B -->|Yes| F{"Can resolve DNS?"}
 F -->|No| G["DNS issue"]
 F -->|Yes| H{"Port open? (nc/telnet)"}
 H -->|No| I["Layer 4: Firewall or service down"]
 H -->|Yes| J{"App responds correctly?"}
 J -->|No| K["Layer 7: Application issue"]
 J -->|Yes| L["Client-side issue"]

ping — Basic Connectivity#

Tests Layer 3 (IP) reachability using ICMP echo:

Cloud Networking

Mon, 01 Jan 0001 00:00:00 +0000

From Physical to Virtual#

Cloud networking uses the same concepts as physical networking (subnets, routing, firewalls) but everything is software-defined and API-driven.

Physical	Cloud Equivalent
Router	Route table
Firewall	Security group / NACL
Switch/VLAN	Subnet
NAT device	NAT Gateway
Physical network	VPC
Leased line	Direct Connect
VPN appliance	VPN Gateway

VPC (Virtual Private Cloud)#

An isolated virtual network within a cloud provider’s infrastructure:

flowchart TD
 subgraph VPC["VPC: 10.0.0.0/16"]
 subgraph AZa["Availability Zone A"]
 PubA["Public Subnet<br/>10.0.1.0/24"]
 PrivA["Private Subnet<br/>10.0.10.0/24"]
 end
 subgraph AZb["Availability Zone B"]
 PubB["Public Subnet<br/>10.0.2.0/24"]
 PrivB["Private Subnet<br/>10.0.11.0/24"]
 end
 end
 
 IGW["Internet Gateway"] --> PubA
 IGW --> PubB
 PubA --> NATGW["NAT Gateway"]
 NATGW --> PrivA
 NATGW --> PrivB
 Internet["Internet"] --> IGW

VPC Design Decisions#

Decision	Recommendation
CIDR size	/16 (65K IPs) — room to grow
Subnet size	/24 (251 usable in AWS) per AZ per tier
AZ count	Minimum 2 for high availability
Tiers	Public, Private, Data (3 tiers)

Public vs Private Subnets#

Aspect	Public Subnet	Private Subnet
Route to internet	Via Internet Gateway	Via NAT Gateway
Inbound from internet	Yes (if SG allows)	No
Public IP	Auto-assigned or Elastic IP	None
Contains	Load balancers, bastion hosts	App servers, databases

Route Tables#

Every subnet has an associated route table:

Subnetting

Mon, 01 Jan 0001 00:00:00 +0000

What Is Subnetting?#

Subnetting divides a large IP network into smaller, logical sub-networks. It’s one of the most fundamental skills in networking — every network engineer, cloud architect, and system administrator must understand it fluently.

Why Subnet?#

Reason	Explanation
Reduce broadcast domains	Fewer hosts receiving every broadcast = less noise, better performance
Improve security	Isolate sensitive systems (databases, management) from general traffic
Efficient IP allocation	Don’t waste a /16 (65,534 hosts) when you only need 30
Routing efficiency	Smaller, summarizable networks reduce routing table size
Organizational structure	Map network topology to departments, floors, functions

IP Address Structure#

An IPv4 address is 32 bits, written as four octets in dotted-decimal: