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Layer 3 - Network

6 min read

Layer

Moves data between different local networks (inter-networking)

  • Routes packets between different networks (LANs).
  • Uses logical addressing, like IP addresses, instead of physical ones (like MAC addresses).
  • Enables data to travel across the internet or between distant networks.

limitations

  • every packet is single & isolated!
  • doesn’t know which program or app on the destination device the packet is meant for
    • (e.g., browser, Discord, Zoom)
  • no flow control
    • if the source transmits faster than the destination can receive, it can saturate the destination causing packet loss
  • packets can be delivered out of order
    • no guarantee that the packets will take the same route from source to destination
    • because of network conditions, they could arrive in a different order
  • packets can go missing
    • network outages/conditions which cause temporary routing loops
    • limited number of hops
  • network conditions can cause delay in delivery

P2P Link

A Point-to-Point (P2P) link is a direct connection between two devices (or two routers/networks)

  • Example
    • Ethernet cable between two routers
  • It’s fine for small setups, but not scalable
    • You’d need a new cable or configuration for every pair of connected networks

Why u need layer 3

  • Layer 2 (like Ethernet) is limited to a single LAN
  • You can’t simply connect 2 LANs together if they use different layer 2 protocols (Ethernet vs Wi-Fi, or ATM vs. PPP)
  • Layer 3 solves this by
    • providing a IP address, a universal addressing scheme
    • routing data from 1 LAN to another using routers that operates at layer 3

Internet Protocol (IP)

Definition

A Layer-3 protocol which adds cross-network IP addressing and routing to move data between LANs without direct P2P links

Concepts

  • IP Packets are moved from source to destination across the internet through many intermediate networks
  • Routers (layer 3 devices) move packets of data across different networks
  • Layer 3 IP Packet: Contains the source & destination IP addresses
  • Layer 2 Frame: Wraps the IP packet and includes the source & destination MAC addresses
  • What happens when the packet travels across networks
    • As it goes from router to router, the Layer 2 frame is stripped and rebuilt at each hop. Only the IP packet stays the same.
    • At each hop, new MAC addresses are used (source = router’s MAC, destination = next hop’s (next router’s) MAC).
    • But the IP source/destination stays constant.

IP Packets

  • Contains:
    • IP source address (where packet is generated) and IP destination address
    • Protocol field
      • has layer 4 protocol (ex. TCP)
      • the destination will know which layer 4 protocol to pass the data into
    • The data - from layer 4
    • TTL (time to live)
      • the packet will live through many different networks
      • defines how many hops the packet can hop before being discarded
      • used to stop packets looping around forever
      • (in v6 - called Hop Limit)
    • and other fields
  • packets remain the same as they move across networks
  • versions 4 and 6
    • 4 - used for decades
    • 6 - more scalable

IP address

It is what identifies a device which uses layer 3 IP networking

  • either statically assigned by humans (static IP) or assigned automatically by machines (DHCP)
    • DHCP - dynamic host configuration protocol
  • IP addresses need to be unique

Subnet Masks

  • Used to determine which part of an IP address is the network address and which part is the host address
  • Devices use subnet masks to check if a destination is on the same network or if it must forward to a gateway
  • steps to determine network/host portions
    • Convert the IP and subnet mask to binary.
    • Perform a bitwise AND between the IP and the subnet mask.
    • The result is the network address
  • Network address (starting address):
    • Network bits + all 0s in host portion
  • Broadcast address (ending address):
    • Network bits + all 1s in host portion

IP gateway

  • the IP address on the local network that packets are forwarded to when the destination is not within the same network
    • When a device needs to communicate with a device outside its own subnet, the data will be sent to the gateway
    • acts as the exit point for any data leaving the local network

IPv4

  • Dotted-decimal notation 4 * 0-255
    • each number is a 8-bit binary number
  • 133.33.3.7
    • network part (133.33)
      • states which IP network this IP address belongs to
    • host part (3.7)
      • represents hosts on the network
      • 3.7 in this case is one device in the network (ex. a laptop)
  • If the “Network” part of 2 IP addresses match, they’re on the same IP network
    • if match → devices are local
    • if not match → devices are remote
    • using submasks

IPv6

Routing

  • The routing layer
    • Layer that routers use to determine how to forward traffic
    • It figures out the path data should take across different networks.
    • It uses IP (Internet Protocol) addresses (not MAC addresses anymore).
  • When the packets are leaving, it’s forwarding it at layer 2, it’s packed in a frame
    • in this ex) the frame has the AWS’s MAC address as its destination

Route Tables & Routes

  • Routing table:
    • A collection of routes used to determine where to forward IP packets
  • Packet forwarding process
    • When a packet arrives, the router checks the destination IP
    • It looks for matching routes in the routing table
    • If multiple routes match, the router chooses the most specific route (i.e., the one with the longest prefix)
  • Route specificity
    • The higher the number after the slash, the more specific the route.
      • /x → first x bits are for the network
      • /0 is the least specific (matches everything — “default route”)
      • /32 is the most specific (matches only one IP)
  • default route
    • 0.0.0.0/0

Address Resolution Protocol (ARP)

Arp

ARP is used to map a Layer 3 IP address to a Layer 2 MAC address

  • When a device wants to send a packet to another IP on the same local network, it needs to encapsulate the packet in a frame
  • But to send a frame, it must know the destination MAC address.
  • If it doesn’t know the MAC address for a given IP, it uses ARP to find it.
  • between layer 2 and 3

Fragmentation

  • if needed, layer 3 breaks packets into smaller pieces (fragments) so it can travel across networks that have size limits
  • Fragments frames to traverse different networks

Example - Routing

Graph View

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