Networking Fundamentals —
Protocols, Routing & VLANs

The Big Picture

Parts 1 and 2 covered the first two layers of networking: the Data Link Layer (NICs, Ethernet, Switches, Bridges) and the Network Layer (IP Addresses, Subnet Masks, Gateways, DHCP). Together they allow devices to communicate locally and across different networks.

Part 3 goes deeper — into the protocols that enable reliable communication (TCP/IP), address resolution (ARP), address translation (NAT), intelligent routing, logical network segmentation (VLANs), and finally the full end-to-end journey a packet takes when you visit google.com.

The TCP/IP Stack

TCP/IP (Transmission Control Protocol / Internet Protocol) is the set of rules that allows devices anywhere in the world to communicate. Think of it as a delivery company — four layers, each with a specific job, working together to move data from an application on your device to a server on the other side of the world.

Application Layer

This is where applications live — Chrome, Firefox, WhatsApp, email clients, file transfer tools. The Application Layer decides what information should be sent. When you open google.com, your browser creates an HTTP request (GET / HTTP/1.1) and hands it to the layer below.

Transport Layer

The Transport Layer is responsible for reliability, port numbers, data segmentation, and error recovery. It breaks application data into smaller segments (or datagrams for UDP), adds port numbers so the receiving OS knows which application to deliver to, and handles retransmission of lost data (TCP only).

TCP — Transmission Control Protocol

TCP is reliable. It guarantees delivery, correct order, error checking, and retransmission of lost data. Before any data transfers, TCP establishes a connection using the Three-Way Handshake:

Use TCP when: reliability is critical, data accuracy matters. Examples: web browsing, email, file transfer.

UDP — User Datagram Protocol

UDP is faster than TCP but provides no guarantees — no delivery confirmation, no ordering, no retransmission. Think of it as throwing a message over a wall: fast, but no confirmation it arrived. Use UDP when: speed and low latency are critical and some data loss is acceptable. Examples: online gaming, voice/video calls, live streaming, DNS queries.

Ports

Many applications share one IP address. The OS needs a way to know which application should receive incoming data. Ports solve this — they identify specific applications or services on a device. Think of an IP address as an apartment building and a port as the apartment number.

Example: 192.168.1.10:443 — building = IP, apartment = port 443 (HTTPS). Ports live at the Transport Layer and are used by both TCP and UDP.

Common Ports

ARP — Address Resolution Protocol

ARP is one of the most important protocols inside LANs. The problem it solves: you know the destination’s IP address, but Ethernet requires a MAC address to actually deliver the frame. ARP answers the question: “Who owns this IP address?”

The ARP process has three steps: (1) PC broadcasts “Who has 192.168.1.20?” to everyone, (2) the target replies with its MAC address via unicast, (3) communication begins. The result is stored in the ARP cache to avoid repeating the broadcast for every packet.

NAT — Network Address Translation

Your home devices use private IP addresses (192.168.x.x) that cannot travel across the Internet. NAT, running on your router, translates private addresses to the router’s public IP when traffic leaves your network — and translates incoming replies back to the correct private device.

Routing

Routing means determining the best path for packets to travel from source to destination. When you access google.com, your packet hops through multiple routers: Home Router → ISP Router → Regional Router → Backbone Router → Google. At each step, the router asks: “Where should this packet go next?”

What is a Router?

A router connects different networks. Unlike switches that ask “Which port?” (based on MAC), routers ask “Which network?” (based on IP). Routers make forwarding decisions using IP addresses and routing tables and perform four key functions: connect different networks, determine the best path, forward packets based on IP, and reduce broadcast domains.

Switching vs Routing

VLAN — Virtual Local Area Network

One of the most important enterprise networking concepts. VLANs allow one physical switch to behave like multiple logical switches. Without VLANs on a switch with HR, Finance, and Engineering departments: everyone is on the same Layer 2 network, broadcasts reach everyone, ARP reaches everyone, and traffic is less isolated.

With VLANs: assign VLAN 10 = HR, VLAN 20 = Finance, VLAN 30 = Engineering. Each VLAN is a separate broadcast domain. Frames are forwarded only within the same VLAN. Traffic between VLANs requires a router (Layer 3 device). Result: logical segmentation on one physical switch.

Access Ports, Trunk Ports & VLAN Tagging

Access Ports

An access port belongs to exactly one VLAN. It connects end devices (PCs, printers, phones) to the switch. Frames leaving an access port are untagged — the device has no idea it’s part of a VLAN.

Trunk Ports

A trunk port carries multiple VLANs over a single physical cable — used to connect switches together. Traffic on a trunk contains VLAN tags so the receiving switch knows which VLAN each frame belongs to.

VLAN Tagging — IEEE 802.1Q

VLANs use the IEEE 802.1Q standard. When traffic leaves an access port and enters a trunk, the switch inserts a 4-byte tag into the Ethernet frame containing the VLAN ID. This tag tells other switches which VLAN owns the frame. When the frame reaches its destination access port, the tag is removed.

Broadcast Domain

A broadcast domain is a group of devices that receive each other’s broadcasts. Without VLANs, an entire switch is one broadcast domain — every device receives every broadcast. With VLANs, each VLAN becomes its own separate broadcast domain, reducing unnecessary traffic and improving security.

How a Packet Travels to Google

This is the complete end-to-end journey that combines everything from Parts 1, 2, and 3:

Part 3 Summary