Chapter 6: Network Layer

Chapter 6: Network Layer

1. The Road Less Traveled…

   • Source address
   • Destination address
   • Sequence number

2. The Network Layer uses four basic processes

   • Addressing end devices
   • Encapsulation
   • Routing
   • De-encapsulation

3. The Network Layer

   • Addressing end devices.
     • End devices must be configured with a unique IP address for identification on the network.
   • Encapsulation
     • The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet.
   • Routing
     • The network layer provides services to direct packets to a destination host on another network.
   • De-encapsulation
     • When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet.

4. Network Layer Protocols

   • Internet Protocol version 4 (IPv4)
   • Internet Protocol version 6 (IPv6)

5. Characteristics of IP

   • Source IP Address and Destination IP Address
     • Connectionless
       • No connection with the destination is established before sending data packets
     • Best Effort
       • IP is inherently unreliable because packet delivery is not guaranteed
     • Media Independent
       • Operation is independent of the medium (i.e., copper, fiber optic, or wireless) carrying the data

6. Connectionless

   • IP is connectionless, meaning that no dedicated end-to-end connection is created before data is sent.

7. IP - Media Independent

   • IP operates independently of the media that carry the data at lower layers of the protocol stack.

8. IPv4 Packet Header

   • An IPv4 packet header consists of fields containing important information about the packet.
     • Significant fields in the IPv4 header include
       • Version
       • Differentiated Services or DiffServ (DS)
       • Time-to-Live (TTL)
       • Protocol
       • Source IPv4 Address
       • Destination IPv4 Address

9. Introducing IPv6

   • Improvements that IPv6 provides include
     • Increased address space
       • IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits.
     • Improved packet handling
       • The IPv6 header has been simplified with fewer fields.
     • Eliminates the need for NAT
       • With such a large number of public IPv6 addresses, NAT between a private IPv4 address and a public IPv4 is not needed.

10. IPv6 Packet Header

    • The fields in the IPv6 packet header include
      • Version
      • Traffic Class
      • Flow Label
      • Payload Length
      • Next Header
      • Hop Limit
      • Source IPv6 Address
      • Destination IPv6 Address

11. Host Forwarding Decision

    • Another role of the network layer is to direct packets between hosts. A host can send a packet to
      • Itself
        • A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1, which is referred to as the loopback interface.
      • Local host
        • This is a host on the same local network as the sending host.
      • Remote host
        • This is a host on a remote network.

12. Host Routing Tables

    • On a Windows host, the route print or netstat -r command can be used to display the host routing table.
      • Entering the netstat -r command or the equivalent route print command, displays three sections related to the current TCP/IP network connections
        • Interface List
          • Lists the Media Access Control (MAC) address and assigned interface number of every network-capable interface on the host, including Ethernet, Wi-Fi, and Bluetooth adapters.
        • IPv4 Route Table
          • Lists all known IPv4 routes, including direct connections, local network, and local default routes.
        • IPv6 Route Table
          • Lists all known IPv6 routes, including direct connections, local network, and local default routes.

13. Router Packet Forwarding Decision

    • When a host sends a packet to another host, it will use its routing table to determine where to send the packet.
      • The routing table of a router can store information about
        • Directly-connected routes
          • These routes come from the active router interfaces.
        • Remote routes
          • These routes come from remote networks connected to other routers.
        • Default route
          • Like a host, routers also use a default route as a last resort if there is no other route to the desired network in the routing table.

14. Directly Connected Routing Table Entries

    • When a router interface is configured with an IPv4 address, a subnet mask, and is activated, the following two routing table entries are automatically created
      • C
        • Identifies a directly-connected network. Directly-connected networks are automatically created when an interface is configured with an IP address and activated.
      • L
        • Identifies that this is a local interface. This is the IPv4 address of the interface on the router.

15. A Router is a Computer

    • Branch
      • Teleworkers, small businesses, and medium-size branch sites.
    • WAN
      • Large businesses, organizations, and enterprises.
    • Service Provider
      • Large service providers. Includes Cisco ASR, Cisco CRS-3 Carrier Routing System, and 7600 Series routers.

16. Router Memory

    • RAM
    • ROM
    • NVRAM
    • Flash

17. LAN and WAN Interfaces

    • Console
      • This is a physical management port that provides out-of-band access to a Cisco device.
    • Secure Shell (SSH)
      • SSH is a method for remotely establishing a secure CLI connection through a virtual interface, over a network.
    • Telnet
      • Telnet is an insecure method of remotely establishing a CLI session through a virtual interface, over a network.

18. Router Bootup Process

    • There are three major phases to the bootup process.
      • Perform the POST and load the bootstrap program.
      • Locate and load the Cisco IOS software.
      • Locate and load the startup configuration file or enter setup mode.

19. Configure the interface

    • Interface type-and-number
    • Description
    • IP address
    • no shutdown