Local Area Network

This page provides introduction to Local Area Network.

Overview

It is a network contained with relatively small area. For example office network. Routers are used to connect LANs.

Media Access Control (MAC) Address

• Mac address is $6$-byte ($48$-bit) physical address assigned to device when it is made.
• It is also know as Burned-In-Address.
• It is globally unique.
• The first $3$ bytes are the OUI(organizationally unique identifier), which is assigned to the company making the device.
• The last $3$ bytes are unique to the device itself.

Ethernet LAN Switching

Let's say we have ethernet setup as shown in below diagram:

Now, suppose PC$1$ wants to send data to PC$3$. The Ethernet frame will include the following MAC and IP addresses:

Source IP: 192.168.1.01
Destination IP: 192.168.1.03
Source MAC: 002F.B012.1001
Destination MAC: FFFF.FFFF.FFFF # This is broadcast MAC address

The destination MAC address is initially unknown because PC$1$ does not know PC$3$'s MAC address. However, since switches are layer $2$ devices that operate using MAC addresses, PC$1$ must first learn PC$3$'s MAC address.

ARP Process

To learn the MAC address, PC$1$ uses the Address Resolution Protocol (ARP). It sends an ARP request, and PC$3$ responds with an ARP reply. The ARP request is a broadcast message sent to all hosts on the network, while the ARP reply is a unicast message sent only to PC$1$.

Below are the steps involved in ARP process:

• PC$1$ sends an ARP request on its interface (G0/1), which is received by Switch $1$.

• Upon receiving the ARP request, Switch $1$ adds PC$1$'s MAC address to its MAC address table. This entry is called a dynamic MAC address because it is learned automatically.

• Since the destination MAC address is a broadcast address (FFFF.FFFF.FFFF), Switch $1$ forwards the ARP request to all its network interfaces except the one on which it was received.

• PC$2$ receives the ARP request but discards it since the destination IP does not match its own.

• Switch $2$ receives the ARP request and adds PC$1$'s MAC address to its MAC address table.

• Since the destination MAC address is a broadcast, Switch $2$ also forwards the ARP request to all its network interfaces except the one it came from.

• PC$4$ discards the frame for the same reason as PC$2$.

• PC$3$ recognizes that the destination IP matches its own. It processes the ARP request and sends an ARP reply frame with the following details:

Source IP: 192.168.1.03
Destination IP: 192.168.1.01
Source MAC: 002F.B012.2001
Destination MAC: 002F.B012.1001

• Switch $2$ receives the ARP reply, updates its MAC address table with PC$3$'s MAC address, and forwards the reply to Switch $1$.

• Switch $1$ forwards the ARP reply to PC$1$ since it knows that PC$1$'s MAC address is associated with the G0/1 interface.

• PC$1$ receives the ARP reply and stores an entry for PC$3$ in its ARP table, associating PC$3$'s IP address with its MAC address.

• PC$1$ can now use this information to send data directly to PC$3$.