Packet Tracer Note

Basic Network Devices

An end system is a device located at the edge of a network, such as a PC or a server. It is the device that actually sends and receives data.

A switch connects end systems inside the same local area network (LAN). It checks the MAC address in an incoming frame and decides which port should receive the frame.

A router connects different networks. It uses the destination IP address to choose the best path and forwards packets according to its routing table.

How a Switch Works

The key address used by a switch is the MAC address. When a switch receives data, it learns the source MAC address and checks the destination MAC address to decide where to forward the frame.

1. Learning
   • The switch records the source MAC address of the received data in its MAC address table.
   • For example, if a PC sends data through port Fa0/1, the switch remembers that the PC’s MAC address is reachable through Fa0/1.
2. Forwarding
   • The switch checks the destination MAC address in the MAC address table.
   • If the destination MAC address is already in the table, the switch forwards the frame only through the correct port.
   • This improves network efficiency because the switch does not send the frame to unnecessary ports.
3. Flooding
   • If the destination MAC address is not in the MAC address table, the switch forwards the frame out all ports except the port that received it.
   • This happens when the switch has not yet learned the destination device.

In short, a switch forwards data inside a LAN based on its MAC address table. At first, flooding may happen because the switch does not know every address yet. As communication continues, the MAC address table fills up and forwarding becomes more precise.

How a Router Works

The key address used by a router is the IP address. A switch forwards frames inside a LAN using MAC addresses, while a router forwards packets between different networks using IP addresses.

1. Routing
   • Routes are added to the routing table through static routing or dynamic routing.
   • Static routing is configured manually by an administrator.
   • Dynamic routing allows routers to exchange routing information and learn paths automatically.
2. Packet Switching
   • The router checks the destination IP address of the received packet.
   • It forwards the packet according to the path listed in the routing table.

How Hosts Receive IP Addresses

A host needs an IP address before it can communicate on a network. There are two common ways to assign an IP address.

Static configuration means the user or administrator manually enters the IP address, subnet mask, and default gateway. This is often used for servers or network devices that should keep the same address.

Dynamic configuration means the host receives an IP address automatically from a DHCP server. This is useful for regular PCs and laptops because it makes address management easier.

Encapsulation and De-Encapsulation

Encapsulation is the process of adding headers to data so it can travel across the network. As data moves down the network layers, headers such as TCP/UDP, IP, and Ethernet headers are added.

De-encapsulation is the reverse process. The receiving device removes headers step by step until it reaches the original data.

Packet Tracer is helpful because I can visually see that devices do not simply send raw data. Each layer adds or removes information as the packet moves through the network.

Practice Screenshot

My first network topology created in Packet Tracer

Router Interface Configuration Flow

For the router to connect both networks, each router interface needs an IP address and must be enabled.

Router> enable
Router# configure terminal

Router(config)# interface fastEthernet 0/0
Router(config-if)# ip address 192.168.10.1 255.255.255.0
Router(config-if)# no shutdown

Router(config-if)# exit
Router(config)# interface fastEthernet 0/1
Router(config-if)# ip address 192.168.20.1 255.255.255.0
Router(config-if)# no shutdown

The no shutdown command turns on the interface. In Packet Tracer, if the triangle near the router port is red, the interface may be shut down or the link may not be fully active yet. After setting the IP address, entering no shutdown activates the port.

Important Host Settings

Each PC or server needs the correct IP address, subnet mask, and default gateway.

Inside the same LAN, the switch forwards traffic based on MAC addresses. However, when a host needs to reach a different LAN, it sends the packet to its default gateway. If the default gateway is wrong, communication with other networks will fail.

For example, if the PC 192.168.10.100 wants to reach 192.168.20.100, the traffic follows this path:

192.168.10.100
-> 192.168.10.1 gateway
-> router
-> 192.168.20.0/24 network
-> 192.168.20.100

Verification Commands

After finishing the configuration, I can verify the router status with these commands:

Router# show ip interface brief
Router# show running-config
Router# show ip route

• show ip interface brief: checks each interface IP address and up/down status
• show running-config: checks the current active configuration
• show ip route: checks the networks known by the router

Finally, I can use ping from a PC to verify communication between different networks.

ping 192.168.20.100

Simulation Screenshot

Packet Tracer simulation mode showing ARP and ICMP traffic

VLAN

Today I reviewed VLAN fundamentals. A VLAN, or Virtual LAN, is a way to divide one physical switched network into multiple logical networks. Even if devices are connected to the same physical switches, VLANs can separate them as if they were on different networks.

The key idea is:

One VLAN = one broadcast domain = one logical network

Traffic that belongs to a specific VLAN is forwarded only through ports that belong to that VLAN. For example, VLAN 10 traffic stays inside VLAN 10, and VLAN 20 traffic stays inside VLAN 20 unless routing is configured between them.

How VLANs Work

A switch still performs the same basic switching functions: learning, forwarding, and flooding. The difference is that it also checks the VLAN number.

When a switch learns a MAC address, it does not only record the MAC address and port. It also records the VLAN ID.

Example MAC address table:

Vlan   Mac Address   Type      Ports
10     AAA           DYNAMIC   Fa0/2
20     BBB           DYNAMIC   Fa0/1
20     CCC           DYNAMIC   Fa0/3
10     DDD           DYNAMIC   Fa0/4

This means that the same switch can keep separate forwarding information for different VLANs. A frame in VLAN 10 is forwarded only within VLAN 10. A frame in VLAN 20 is forwarded only within VLAN 20.

VLAN Range

VLAN ranges are important because some VLAN IDs are reserved.

• VLAN 1 is the default VLAN and cannot be deleted.
• VLANs 2 through 1001 can be added, changed, or deleted.
• VLANs 1002 through 1005 are reserved for Token Ring and FDDI.

Advantages of VLANs

VLANs reduce unnecessary network traffic because broadcast frames are limited to a smaller broadcast domain. Examples of broadcast traffic include ARP requests and NetBIOS name queries.

VLANs also reduce security risk. If a worm, virus, or noisy broadcast spreads in one VLAN, the damage is more contained because the broadcast does not automatically reach every device in the physical network.

Main benefits:

• Reduced broadcast traffic
• Smaller broadcast domains
• Better logical separation
• Easier network management
• Reduced chance that a broadcast problem affects the entire switched network

Access Ports

An access port carries traffic for a single VLAN. It is usually used for interfaces connected to end devices such as PCs, printers, and servers.

Trunk Ports

A trunk port carries traffic for multiple VLANs over one physical link. Trunk ports are commonly used between switches or between a switch and a router.

Without trunking, separate physical links would be needed for each VLAN. With trunking, VLAN 10 and VLAN 20 traffic can both travel across the same link while still remaining logically separate.

Basic trunk configuration:

Switch# configure terminal
Switch(config)# interface fastEthernet 0/3
Switch(config-if)# switchport mode trunk

The switch must be able to identify which VLAN each frame belongs to as it crosses the trunk link. That is the role of a trunking protocol.

Trunking Protocols

Two trunking protocols were covered:

IEEE 802.1Q

802.1Q is the standard trunking protocol. It identifies VLAN traffic by inserting a tag field into the Layer 2 frame header. This tag tells the receiving switch which VLAN the frame belongs to.

ISL

ISL is Cisco proprietary. It identifies VLAN traffic by adding an ISL header and ISL FCS around the original frame. It is older and less commonly used than 802.1Q.