Networking Concepts: Fragmentation and TTL

Fragmentation Offset

• Data packets are fragmented into smaller pieces by the source computer
• Fragmentation offset values determine the position of each fragment within the original data packet
• The source computer includes these values so the destination computer can reassemble the fragmented packets in the correct order
• Fragmented packets travel through different routes to a destination computer
• Fragmented packets may be fragmented further at an intermediate network
• At the destination, the fragmented packets are reassembled using the offset values
• This process is called defragmentation

Time to Live (TTL)

• IP packets travel through many routers in a network
• Each router routes the packet based on the routing table
• If there is a routing table issue, the packet can be sent in the wrong direction
• Routing problems can lead to packets getting lost or stuck in a loop
• The TTL field in an IP packet helps prevent this by decrementing the value with each hop the packet takes
• When the TTL value reaches 0, the packet is discarded and an ICMP Time Exceeded message is sent back to the source

IP Addressing Classes

• Class A: Uses 8 bits for the network ID and 24 bits for the host ID
• Class B: Uses 16 bits for the network ID and 16 bits for the host ID
• Class C: Uses 24 bits for the network ID and 8 bits for the host ID

Network and Broadcast Address

• Network address: The Host ID portion of the IP address is considered as all 0s
• Broadcast address: The Host ID portion of the IP address is considered as all 1s
• Example: 103.58.35.1
• Network Address: 103.0.0.0
• Broadcast Address: 103.255.255.255

IPv6 Address - Omitting Leading Zeros

• Leading zeros in any 16-bit section or hextet can be omitted for a shorter notation
• Example:
  • 01AB can be represented as 1AB
  • 09F0 can be represented as 9F0
  • 0A00 can be represented as A00
  • 00AB can be represented as AB

IPv6 Address - Omitting All 0 Segments

• A double colon (::) can replace any single, contiguous string of one or more 16-bit segments (hextets) consisting of all 0s
• Double colon (::) can only be used once within an address to avoid ambiguity
• This is known as the compressed format
• Example:
  • 2001:0DB8::ABCD:0000:0000:1234
  • 2001:0DB8::ABCD:0000:0000:0000:1234
  • 2001:0DB8:0000:ABCD::1234
  • 2001:0DB8:0000:0000:ABCD::1234

IPv6 Address Types

• Unicast: One-to-one communication, where a packet is sent from a single source to a single destination
• Multicast: One-to-many communication, where one source broadcasts to multiple destinations
• Anycast: One-to-nearest communication, where a packet is sent to the closest device among multiple destinations with the same address
• IPv6 does not have broadcast addresses.

IPv6 Unicast Addresses

• Global Unicast: Globally unique and Internet-routable addresses. Similar to public IPv4 addresses.
• Link-Local: Used to communicate with devices on the same local link
• Unique Local: Private addresses like IPv4 private addresses, but with significant differences
• 2001:0DB8::/32 address has been reserved for documentation purposes.

Host Configuration

• Manually configuring IPv6 addresses is similar to configuring IPv4 addresses
• The default gateway address for PC1 is 2001:DB8:ACAD:1::1, which is the global unicast address of the router's GigabitEthernet interface on the same network

Dynamic Routing Scenario

• Network changes are updated automatically using routing protocols
• Routing tables are initially populated with connected and static records
• Dynamic updates occur automatically to adapt to changing network conditions

Dynamic Routing - Advantages

• Less work for administrators maintaining configuration
• Automatic updates based on network topology changes
• Reduced configuration errors
• Suitable for scalable and growing networks

Dynamic Routing - Disadvantages

• Router resources are used (CPU cycles, memory, and bandwidth)
• Requires deeper administrator knowledge for configuration, verification, and troubleshooting

Routing Protocols Features

• Network changes are automatically updated in routing tables for all routers
• Routing protocols select the best route when multiple options exist for a destination
• Traffic is distributed across different routes to optimize performance

What is a Routing Protocol?

• A set of rules and procedures by which routers share network information with each other
• Allows for optimal communication between devices in a network

IP Packet Header

• Contains essential information for routing and processing IP packets
• Key fields include:
  • Source IP Address: The IP address of the source device
  • Destination IP Address: The IP address of the destination device
  • TTL: Time to live field to prevent packets from looping indefinitely
  • IP Options: Optional fields for special functionality

Switch

• Intelligent network device
• Operates at Layer 2 (Layer 2 switch) or Layer 3 (Layer 3 switch)
• Layer 2 switches work at the data link layer, forwarding frames based on MAC addresses.
• Layer 3 switches work at the network layer and support routing.

Switch Form Factors

• Fixed Configuration
• Modular Configuration
• Stackable Configuration

Business Considerations for Switch Selection

• Cost: Considerations include interface speed, number of interfaces, supported features, and expansion capabilities.
• Port Density: Number of devices on the network.
• Power: Considerations include power access points, PoE, and redundant power supply.
• Reliability: 24/7 continuous access.
• Port Speed: Ethernet, FastEthernet, GigabitEthernet.
• Scalability: Future network growth.

Switch Functions

• Address Learning: Switches learn the MAC addresses of devices connected to their ports.
• Forward/Filter Decisions: Switches use the learned MAC addresses to forward frames to the correct destination port.
• Loop Avoidance: Prevents network loops by using Spanning Tree Protocol (STP) or other mechanisms.

MAC Address Table

• Layer 2 switches and bridges use a MAC address table to remember the source MAC addresses of frames received on each interface.
• The MAC address table helps switches determine how to forward frames to specific devices connected to their ports.
• The switch uses the learned information in the MAC address table to send frames to the correct destination port, effectively creating a connection between the source and destination devices.