Osi on Aperture Zone

RJ45: The Complete Guide — Standards, Cable Categories, Connectors, and Tools

Sun, 12 Jul 2026 00:00:00 +0000

This article is in response to a request from @Menvé, a site member and cybersecurity instructor: his students were looking for a clear and comprehensive guide on the RJ45. Because we often forget—especially when we’re just starting out in cybersecurity—but it all starts here: the physical layer is the foundation on which everything else rests. It’s hard to make sense of segmentation, VLANs, or 802.1X without knowing what’s actually traveling through the cables. For segmentation and VLANs, you can also check out the excellent article by @SilenShttps://aperturezone.fr/posts/cisco-firewall-final/

MAC Addresses: The Complete Guide — Layer 2, CAM Table, ARP, and Security

Thu, 16 Jul 2026 00:00:00 +0000

This article builds on the RJ45 guide. That guide covered copper, twisted-pair cables, and crimping—the physical layer. Here, we’re moving up a level—just one, but the one everyone skips. Between the cable and the IP address, there’s an entire layer that most people pass through without even noticing it: Layer 2, the layer of MAC addresses.

And that’s a shame, because that’s where a good half of everyday network incidents are resolved, and where attacks that haven’t aged a bit in thirty years play out.

Layer 4: TCP, UDP, and Ports — The Complete Guide

Fri, 31 Jul 2026 00:00:00 +0000

Fourth floor, and a change of pace. So far, everything we've built—the cable, MAC addresses, IP and routing—share a common premise, the one on which the previous article ended: the network makes no promises. An IP packet can be lost, duplicated, or arrive out of order. IP does its best and washes its hands of the rest.

Yet your 4 GB file arrives intact. Your web page displays in the correct order. Your apt upgrade never downloads a half-corrupted package without noticing it.

Layers 5 and 6: The Phantom Layers

Sun, 02 Aug 2026 00:00:00 +0000

Fifth and sixth layers. After the cable, the MAC addresses, the IP, and TCP, logically, we should be talking about the session layer, followed by the presentation layer.

Except that we have to be honest: these two layers are practically nonexistent.

Not in the sense that they’re useless—but in the sense that, in the network you manage every day, no one implements them as such. There is no “Layer 6 protocol” the way there is a Layer 3 (IP) or Layer 4 (TCP). These are two layers that the OSI model envisaged, that theory describes in detail, and that reality has simply absorbed elsewhere.

Layer 7: The Application — HTTP, DNS, DHCP, and the Top of the Stack

Tue, 04 Aug 2026 00:00:00 +0000

Seventh and final layer. We started with the cable, we went through MAC addresses, IP and routing, TCP and ports, and we found that layers 5 and 6 were phantom layers. Here we are at the top.

And the top is where everyone lives. Layer 7 is what you see: the browser loading a page, an email being sent, a domain name being resolved, the machine receiving its IP address at startup. This is the application layer—the only one the user interacts with directly, and by far the richest in protocols.

Firewalls and the OSI Model: What a Firewall Actually Reads in Your Packets

Fri, 14 Aug 2026 00:00:00 +0000

Un pare-feu ne « bloque » rien en bloc : il lit des en-têtes, couche par couche, et chaque couche autorise un type de filtrage différent. Tour d’horizon de la couche 2 à la couche 7, avec pfSense comme cas d’école.

OSI now has its own section

Fri, 31 Jul 2026 00:00:00 +0000

Those who follow aperturezone saw this coming: ever since the RJ45 guide, network articles have been piling up layer by layer. It was time to give them a home.

A Dedicated Section

The OSI model now has its own section: aperturezone.fr/osi. There, we break down the network layer by layer, from the cable all the way to the application—one article per layer, always tailored for those preparing for the TSSR or wanting to understand what’s really going on under IP.

The OSI Model, Layer by Layer

Sat, 18 Jul 2026 00:00:00 +0000

This series breaks down the network layer by layer, from the cable all the way to the application. Seven layers, six articles, always the same promise: no theory for theory’s sake. Each layer comes with the commands that go with it, the failures it causes, and how to diagnose them—all tailored for those preparing for the TSSR professional certification or who simply want to understand what’s really going on under IP.