हिंदी में: कंप्यूटर आपस में डेटा कैसे भेजते हैं — OSI मॉडल, IP एड्रेस, TCP/UDP और इंटरनेट प्रोटोकॉल।
What is a computer network?
A computer network is a group of devices connected to share data and resources. The internet is a “network of networks” connecting billions of devices.
By size: PAN (personal, e.g. Bluetooth), LAN (a building or campus), MAN (a city), WAN (countries — the internet).
Topologies: bus, star (most common in LANs), ring, mesh (highly reliable), tree and hybrid.
Devices: NIC, hub (layer 1), switch (layer 2, uses MAC addresses), router (layer 3, uses IP addresses), modem, access point, firewall.
The OSI model (7 layers)
| # | Layer | Job | Examples |
|---|---|---|---|
| 7 | Application | Services for user applications | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Data format, encryption, compression | TLS/SSL, JPEG |
| 5 | Session | Opens, manages and closes sessions | RPC, NetBIOS |
| 4 | Transport | Process-to-process delivery, reliability | TCP, UDP |
| 3 | Network | Logical addressing and routing | IP, ICMP |
| 2 | Data link | Framing, MAC addressing, error detection | Ethernet, Wi-Fi (802.11) |
| 1 | Physical | Bits on the medium | Cables, fibre, radio |
The practical TCP/IP model has four layers: Application, Transport, Internet and Link (network access).
IP addressing
- IPv4 – 32 bits, written as four decimal numbers (e.g.
192.168.1.10), about 4.3 billion addresses. - IPv6 – 128 bits, written in hexadecimal (e.g.
2001:db8::1), a practically unlimited address space.
Private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16. NAT lets many private devices share one public IP.
Subnetting example
192.168.1.0/26 means 26 network bits and 6 host bits.
– Subnet mask: 255.255.255.192
– Addresses per subnet: 2⁶ = 64; usable hosts: 64 − 2 = 62
– Subnets: .0, .64, .128, .192
TCP vs UDP
| TCP | UDP | |
|---|---|---|
| Connection | Connection-oriented (3-way handshake: SYN, SYN-ACK, ACK) | Connectionless |
| Reliability | Guaranteed, ordered delivery with acknowledgements and retransmission | Best effort; no guarantee |
| Control | Flow control (sliding window) and congestion control | None |
| Speed | Slower, more overhead | Faster, lightweight |
| Used by | Web (HTTP/1.1, HTTP/2), email, file transfer | DNS queries, video calls, online games, streaming |
Note: HTTP/3 runs over QUIC, which is built on UDP.
Important protocols and ports
| Protocol | Purpose | Port |
|---|---|---|
| HTTP / HTTPS | Web pages | 80 / 443 |
| DNS | Converts domain names into IP addresses | 53 |
| DHCP | Automatically assigns IP addresses | 67/68 |
| SMTP | Sending email | 25 (587 for submission) |
| IMAP / POP3 | Receiving email | 143 / 110 |
| FTP | File transfer | 20/21 |
| SSH | Secure remote login | 22 |
ARP maps an IP address to a MAC address on a local network. ICMP carries error and diagnostic messages (used by ping and traceroute).
Routing
Routers forward packets using routing tables. – Distance vector (e.g. RIP) – routers share distances with neighbours (Bellman–Ford idea). – Link state (e.g. OSPF) – routers build a full map and run Dijkstra’s algorithm. – BGP – the path-vector protocol that connects networks across the internet.
Error detection and control
Parity bits, checksums and CRC detect errors; Hamming code can also correct single-bit errors. Flow control methods include stop-and-wait, Go-Back-N and Selective Repeat.
What happens when you open a website?
- The browser asks DNS for the IP address of the domain.
- A TCP connection (or QUIC) is opened to the server, then a TLS handshake secures it.
- The browser sends an HTTP request; the server returns an HTTP response (HTML, CSS, JS).
- The browser renders the page and fetches additional resources.
Quick revision
- OSI: Please Do Not Throw Sausage Pizza Away (Physical → Application).
- Switch = MAC (layer 2); router = IP (layer 3).
- TCP is reliable; UDP is fast.
- /26 gives 62 usable hosts.