Introduction to TCP/IP (Part 1) - Routers, Switches and Addressing

Source: Microchip Developer Help

Routers in Local Networks

Routers connect one network to another. They:

Create local networks and control access to them
Route TCP/IP traffic on local networks
Enable local network traffic to reach the Internet using Network Address Translation (NAT)
Use firewalls to restrict public Internet access to the local network

What is an IP Address?

IP addresses uniquely identify every host (network node) on a TCP/IP network.

Virtual addresses assigned by routers
Each of the four 8-bit fields is represented by a decimal number (0-255)
Controlled by a DHCP server running in the router
Assigned IP addresses can change at any time

IPv4 vs IPv6

Version	Address Size	Total Addresses
IPv4	32-bit (4 octets)	~4.3 billion (about 1 per person)
IPv6	128-bit (8 x 16-bit fields)	3.4 × 10³⁸ (5 × 10²⁸ per person)

Obtaining IP Addresses (DHCP Process)

When a device connects to a network, it automatically requests an IP address from the router.

Step 1: PC Generates Request for IP Address

PC’s IP Address is 0.0.0.0 before assignment
PC and router are physically connected via Ethernet cable
Each end of the cable connects to a network PHY (transceiver for generating/receiving signals)
Broadcast IP address: 255.255.255.255
DHCP (Dynamic Host Configuration Protocol) is used to request and grant IP addresses

Step 2: IP Address Request Received in Router

Router receives the broadcast packet
Router sees sender needs an IP address and creates a new one
Other hosts discard the packet once they realize they cannot provide an IP address

Step 3: Router Offers an IP Address to the PC

Router allocates a new IP address and broadcasts a message containing it
Uses broadcast IP because PC doesn’t know its IP yet and can’t filter packets
Frame includes the destination MAC address of the PC (used by switch)

Step 4: PC Receives and Configures IP Address

PC receives the broadcast packet
Opens it and finds the IP address assignment message
Configures its network interface with the assigned IP

Switches in Local Networks

A switch enables connection of multiple devices to the same network.

Each network interface has its own dedicated PHY
Uplink port: Tx and Rx signals are reversed (no crossover cable needed)
Most new switches have Auto-MDIX interfaces (automatically switch Tx/Rx if needed)

Switches Inside Routers

Most home/small business routers have a built-in switch.

Ref: https://labs.iximiuz.com/courses/computer-networking-fundamentals/bridge-vs-switch

Switches use MAC Addresses

Switches use Media Access Controller (MAC) addresses to forward and filter data.

MAC controls Layer 2 network functions
Forwards and filters frames based on MAC addresses

Two Addresses per Network Host

Address Type	Layer	Description
IP Address	Layer 3	Virtual address
MAC Address	Layer 2	Physical address

Key Differences: Switches vs Routers

Device	Layer	Address Type	MAC Address?
Switch	Layer 2	MAC only	No (transparent to network)
Router	Layer 3	IP	Yes - two: LAN and WAN

Switches vs Routers

Gemini

Ref: https://www.conceptdraw.com/examples/block-diagram-of-a-computer-network-with-8-computer-switch-router-1-lan-printer

Ref: https://labs.iximiuz.com/courses/computer-networking-fundamentals/bridge-vs-switch

MAC Functions

Generates frames to send to the network
Receives frames from the network
Frames without matching MAC or broadcast address (FF:FF:FF:FF:FF:FF) are not forwarded to Layer 3
Enables multiple devices to access the same physical network using:
- CSMA/CD - Carrier Sense Multiple Access with Collision Detection (Ethernet)
- CSMA/CA - Carrier Sense Multiple Access with Collision Avoidance (WLAN)

What is a MAC Address?

A MAC (Media Access Control) address is a unique 48-bit identifier assigned to a network interface controller (NIC) for hardware identification on a network.

Fixed hardware-based address that never changes
Programmed during manufacturing
Globally unique (managed by IEEE registration authority)
Six 8-bit fields expressed as hex numbers (e.g., 00:1A:2B:3C:4D:5E)

Switch Operation on a Local Network

A switch uses a routing table to associate port numbers with MAC addresses.

Step 1: PC Sends a Frame to the Switch

Host attempts to communicate with router to obtain an IP address
Creates a packet with broadcast IP address
Encapsulates packet into a frame with broadcast MAC address (FF:FF:FF:FF:FF:FF)

Step 2: Switch Receives Frame

Uses routing table to associate host’s MAC address with the receiving interface
Frame contains source MAC address

Step 3: Switch Broadcasts Frame to All Nodes

MAC sees this is a broadcast frame
Switch forwards frame to all connected hosts

Step 4: Router Sends Reply to PC

Switch associates router’s MAC address with the receiving interface

Step 5: Switch Forwards Frame to PC

Switch looks up destination MAC address in routing table
Forwards frame only to the appropriate port
No other port sees this frame

Example: Simplified Local Network TCP/IP Communication

Scenario

Embedded network device with a web page for monitoring/control
PC accesses the web page via browser

Step 1: Open Web Browser and Enter Development Board’s IP Address

Both PC and development board have IP addresses
Switch’s routing table has been updated with MAC addresses for each port

Step 2: PC Generates and Transmits a Frame

Web browser creates a message requesting the web page at IP 192.168.1.102
Source and destination IP addresses added to create a packet
Source and destination MAC addresses added to create a frame
Frame sent to PHY for transmission

Step 3: Frame is Forwarded Through the Switch

Switch opens frame to find destination MAC address
Finds MAC in routing table, sends to appropriate port
Switch only cares about Layer 2 (MAC) addresses, not IP addresses

Step 4: Frame Arrives at Development Board

Destination MAC address checked → matches device’s MAC
Frame opened, destination IP address checked → matches device’s IP
Packet opened to see message → destined for web server
Message sent to web server application

Filtering at each layer:

MAC mismatch → frame discarded at Layer 2 (Data Link)
IP mismatch → packet discarded at Layer 3 (Network)
Application not running → discarded at Layer 4 (Transport)

Step 5: Web Server Sends the Webpage to the PC

Web server generates message containing the web page
Source/destination IP addresses added → packet
Source/destination MAC addresses added → frame
Frame sent to PHY for transmission
Switch forwards to PC using routing table
PC opens frame, checks MAC, opens packet, checks IP
PC opens message and receives the HTML file

Question: What happens next? Answer:

The frame is received at the switch.
The switch finds the frame’s destination MAC address and uses its routing table to determine what port to forward the frame to.
The frame is sent to the PC
The PC opens the frame and checks the destination MAC address to determine if it needs to pay attention to it.
The PC opens the packet and checks the destination IP address.
The PC opens the message and finds the web page (which is just an HTML file) it requested.