TCP/IP & DNS

Step 1: The TCP/IP Model

The TCP/IP model is the foundational architecture of the internet. It organizes networking into four layers, each with a specific responsibility. Data flows down the stack when sending and up the stack when receiving.

Click each layer below to learn about its role and protocols:

Layer 4: Application HTTP, FTP, SMTP, DNS

Layer 3: Transport TCP, UDP

Layer 2: Internet IP, ICMP, ARP

Layer 1: Network Access Ethernet, Wi-Fi

Click a layer above to see its details.

Key terms: A protocol is a set of rules for communication. A packet is a unit of data. An IP address identifies a device on a network. A port identifies a specific service on that device.

Step 2: IP Addresses & Ports

An IPv4 address is a 32-bit number written as four octets separated by dots. Each octet ranges from 0 to 255. A port number (0-65535) identifies a specific process or service. Together, an IP address and port form a socket address.

Enter an IPv4 Address

Enter a Port Number

Socket Address

192.168.1.1:80

Step 3: TCP Three-Way Handshake

Before data is sent over TCP, the client and server must establish a connection using a three-way handshake. This ensures both sides are ready to communicate and agree on initial sequence numbers.

💻

Client

🖥

Server

▶

SYN

◀

SYN-ACK

▶

ACK

Click "Next Step" to begin the handshake.

Step 4: Data Encapsulation

When you send data over a network, each layer of the TCP/IP stack adds its own header to the data. This process is called encapsulation. Type a message below and watch it get wrapped layer by layer.

Your Message

APPLICATION DATA

Hello, server!

↓ + TCP Header (port)

TCP SEGMENT

Src:443 Dst:80 Data

↓ + IP Header (IP address)

IP PACKET

Src:10.0.0.1 Dst:93.184.216.34 TCP Data

↓ + Ethernet Header (MAC address)

ETHERNET FRAME

MAC:aa:bb:cc:dd:ee:ff IP TCP Data FCS

Step 5: DNS Resolution

The Domain Name System (DNS) translates human-readable domain names into IP addresses. When you type a URL, your browser must resolve the domain through a chain of DNS servers. Enter a domain below and watch the lookup process.

Domain Name

1

Browser Cache
Check if domain was recently resolved

MISS

↓

2

OS Cache
Check operating system DNS cache

MISS

↓

3

Recursive Resolver
ISP or configured DNS resolver (e.g. 8.8.8.8)

QUERY

↓

4

Root Server
Directs to the correct TLD server

REFER

↓

5

TLD Server
Handles .com, .org, .net, etc.

REFER

↓

6

Authoritative Server
Holds the actual DNS record

FOUND

Enter a domain and click "Resolve" to trace the DNS lookup.

Step 6: Knowledge Check

1. Which layer of the TCP/IP model is responsible for routing packets across networks?

Application Layer Transport Layer Internet Layer

2. What are the three steps of the TCP three-way handshake, in order?

ACK, SYN, SYN-ACK SYN, SYN-ACK, ACK SYN, ACK, SYN-ACK

3. Which DNS server holds the actual IP address record for a domain?

Root Server TLD Server Authoritative Server