The Network Layer

On a LAN, nodes can communicate through their physical Mac addresses

• Optimal for small-scale, not large-scale

Characteristics of an IP Address:

• 32-bit long numbers made up of four octets, each being described in decimal numbers, in dotted decimal notation

  • A single octet: 8 bits that can represent all decimal numbers from 0-255

  • Example: 12.34.56.78

  • Non-example: 123.456.789.100 because bolded version is greater than 255

IP addresses are distributed in large sections to organizations and companies, making them more hierarchical and easier to store data about compared to physical addresses

• Ex: IBM owns every IP with the number 9 as the first octet

IP addresses belong to networks, not devices, so your device will connect to different networks with different IP addresses

Dynamic Host Configuration Protocol: connect a new device and an IP address will be assigned to it automatically

• Produces dynamic IP addresses

• Static IP address: configured on a node manually

Ethernet Frames: data packets at the ethernet layer

Describe how IP datagrams are encapsulated inside the payload of an Ethernet frame

• IP Datagram: data packets under the IP protocol

  • Like any ethernet frame, IP datagrams are a highly structured, strictly defined series of fields

• Primary sections: header and payload

Correctly identify and describe the many fields of an IP datagram header

• Header

  • Contains more data than an ethernet frame header

  • 4 bits

  • indicates what version of Internet Protocol is being used

    • Most common versions: version 4 (IPv4)

    • Next most common version: version 6 (IPv6)

• Length Field

  • 4 bits

  • Declares how long the entire header is

  • almost always 20 bytes in length (which is the minimum)

• Service type field:

  • 8 bits

  • can be used to specify details about quality of service, or QoS, technologies

    • QoS: there are more services that allow routers to make decisions about which IP datagram may be more important than

      others

• Total length field

  • 16 bits

  • indicates the total length of the IP datagram it’s attached to

• Maximum size of a single datagram is the largest number you can represent with 16 bits: 65,353

  • Otherwise, IP layer splits data into individual packets, utilizing the identification field

• Identification field:

  • 16 bits

  • groups messages together

  • ensures that the receiving end understands that every packet with the same value in that field is part of the same transmission

• Flag Field:

  • Indicates if a datagram is allowed to be fragmented or if the datagram has already been fragmented

    • Fragmentation: process of taking a single IP datagram and splitting it up into several smaller datagrams

• Fragmentation offset field

  • Contains values used by the receiving end to take all the parts of a fragmented packet and put them back together in the correct order

• Time to live (TTL) field

  • 8 bits

  • Indicates how many router hops a datagram can traverse before it’s thrown away

  • Useful when there’s a misconfiguration in routing leading to an endless loop

• Protocol fields

  • 8 bits

  • contains data about what transport layer is being used

  • Most common transport layer protocols: TCP and UDP

• Header checksum field

  • checksum of the contents of the entire IP datagram header

• Source IP Address

  • 32 bits

• Destination IP address

  • 32 bits

• IP Options Field

  • optional field

  • sets special characteristics for datagrams primarily used for testing purposes

• Padding field: series of zeros to ensure the header is the correct total size

• Encapsulation:

  • Similar to data payload section for Ethernet

  • Entire contents of IP datagram are encapsulated as a packet into the payload for an ethernet frame

Sections of an IP Address:

• Network ID: 1st octet

• Host ID: 2nd octet

Address class system: a way of defining how the global IP address space is split up

• Class A: 1st octet is used for the network ID, last three for the host ID

  • total of 24 bits of host ID space

  • left most bit begins with 0

  • any IP from 0-127 for first octet

• Class B: 1st two octets are used for network ID, second two are used for host ID

  • left most bit begins with 10

  • any IP from 128 - 191

• Class C: 1st three octets are used for network ID, last is used for host ID

  • left most bit begins with 110

  • any IP from 224

Address Resolution Protocol (ARP)

• ARP: protocol used to discover the hardware address of a node with a certain IP address

• Once an IP datagram has been fully formed, it needs to be encapsulated inside an ethernet frame, which means the transmitting device needs a destination MAC address to complete the Ethernet frame header

• ARP table: list of IP addresses and the MAC addresses associated with them

  • if destination lacks an entry in the ARP table, the node that wants to send data sends a broadcast ARP message to the MAC Broadcast addresses

    • These types of broadcast ARP messages are received by all computers on a LAN

  • When ARP broadcast is received, sends an ARP response, which contains the MAC address for the network interface in question

• ARP table entries generally expire after a short amount of time to ensure changes in the network are accounted for