TCP/IP Model

Also Known As

TL;DR

Explanation

The TCP/IP model layers from bottom to top: Link (Ethernet, WiFi — physical transmission), Internet (IP — addressing and routing between networks), Transport (TCP/UDP — end-to-end delivery), Application (HTTP, DNS, SMTP — what applications speak). Each layer adds a header encapsulating the layer above. Understanding TCP/IP explains why HTTP needs TCP (reliable), why UDP suits video (fast, loss-tolerant), and why IP routing works across heterogeneous networks.

Diagram

flowchart TD
    subgraph Application
        HTTP2[HTTP] & DNS2[DNS] & SMTP2[SMTP]
    end
    subgraph Transport
        TCP2[TCP] & UDP2[UDP]
    end
    subgraph Internet
        IP2[IP Routing]
    end
    subgraph Link
        ETH2[Ethernet / WiFi]
    end
    Application -->|Data| Transport -->|Segments| Internet -->|Packets| Link -->|Frames| WIRE[Physical Medium]
style Application fill:#1f6feb,color:#fff
style Transport fill:#238636,color:#fff
style Internet fill:#d29922,color:#fff
style Link fill:#6e40c9,color:#fff

Common Misconception

Why It Matters

Common Mistakes

• Confusing TCP (reliable, ordered, connection-oriented) with UDP (unreliable, unordered, connectionless).
• Not understanding that IP addresses are Layer 3 (Internet) and MAC addresses are Layer 2 (Link) — they serve different scopes.
• Assuming HTTP requires TCP — HTTP/3 uses QUIC over UDP for improved performance.
• Not accounting for TCP connection overhead — each new TCP connection requires a 3-way handshake adding round-trip latency.

Code Examples

# Ignoring TCP connection cost — new connection per request:
# PHP script making 100 HTTP calls with new connections each time:
for ($i = 0; $i < 100; $i++) {
    $ch = curl_init($url);
    curl_exec($ch);
    curl_close($ch); // Connection closed — 100 TCP handshakes
}

# Reuse connections with persistent handles:
$mh = curl_multi_init();
$handles = [];
for ($i = 0; $i < 100; $i++) {
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_2_0); // HTTP/2 multiplexing
    $handles[] = $ch;
    curl_multi_add_handle($mh, $ch);
}
// HTTP/2 multiplexes all 100 requests over one TCP connection

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/HTTP/Overview

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