2.1.4-2.1.6 Layer 3 - Network

Layer 3 - Network - PART 1

• Two geographically separated local area networks (LANs) can be connected to create a common Layer 3 network.
• Isolated Layer 2 networks (LAN1 and LAN2) can communicate within their local network but not outside.
• Point-to-point links can connect networks across distances, but this is expensive and doesn't scale well.
• Layer 2 networks need to use the same Layer 2 protocol for communication, which presents challenges as not everything uses the same protocol.
• Ethernet is a commonly used Layer 2 protocol for local networks.
• Long-distance point-to-point links use protocols, such as PPP (Point-to-Point Protocol), MPLS (Multiprotocol Label Switching), or ATM (Asynchronous Transfer Mode).
• Layer 2 moves frames between local sources and destinations.
• Layer 3 and internetworking are needed to move data between different local networks.
• Layer 3 enables the connection of multiple Layer 2 networks.
• Internet Protocol (IP) gives IP addresses for cross-network communication via routing.
• A router is a Layer 3 device which moves packets of data across different networks, encapsulating packets inside ethernet frames.
• Encapsulation means an IP packet is put inside an ethernet frame for a part of a journey, and then a new frame is added for the next local network.
• Packets contain data, source, and destination addresses.
• The source and destination of frames are generally local, but IP packets' source and destination addresses can be on opposite sides of the planet.
• Packets stay the same during their journey, while frames change every time a packet moves between networks.

Layer 3 - IP - Packet Structure

• There are two versions of the Internet Protocol in use: Version 4 and Version 6 (offering more scalability).
• Packets are similar to frames that contain data being moved, as well as source and destination addresses.
• Source IP address is typically the IP of the device generating the packet.
• Destination IP address is the intended destination IP for the packet.
• Data is generally provided from a Layer 4 protocol.
• The protocol field indicates which protocol is used by Layer 4.
• ICMP, TCP, and UDP are examples of protocols that might be referenced using the protocol field.
• Routers use that field to identify the layer 4 protocol to pass the data into
• Time-to-Live or TTL defines how many hops a packet can move through to stop packets from looping around forever.
• Routers use the source and destination IP address fields to move packets across networks.
• Internet is formed using Layer 2 frames and links

Layer 3 - Network - PART 2

• IP addressing identifies a device using layer 3 IP networking.
• An IP address is a valid IP version 4 address to send packets to a destination.
• Dotted-decimal notation is the typical format of IP addresses (four decimal numbers between 0 and 255 separated by dots).
• All IP addresses consist of a network part and a host part.
• The network part indicates which IP network an IP address belongs to, while the host part represents the host on that network.
• Knowing which part of an IP address is the network and which part is the host determines if two IP addresses are on the same or different IP networks.
• IP addresses are technically binary numbers (with each decimal part being an eight-bit binary number, known as an octet)
• A /16 prefix means the first 16 bits represent the network, with the rest representing the hosts.
• IP addresses are assigned statically by humans (static IP) or automatically by machines (DHCP).
• On a network, IP addresses need to be unique.
• Subnet masks are configured on Layer 3 interfaces and along with IP addresses.
• Default gateway is an IP address on the local network, which packets are forwarded to if the destination is not a local IP address.
• Subnet masks allow an IP device to know if an IP address it's communicating with, is on the same network.
• To use a subnet mask, convert the dotted-decimal notation into a binary number.
• The prefix is the number of ones in the subnet mask starting from the left
• Ones (1) in a subnet mask represent the network, and zeros (0) represent the host component.
• A binary overlay of subnet mask and IP address reveals the network/host.
• Starting address of a network includes the network component of the IP address and all zeros for the host part.
• Ending address includes the network part and all ones for the host.
• Determining whether to send data directly on a local network using subnet masks is how a lot of the intelligence of Layer 3 is used.

Layer 3 - Network - PART 3

• Route tables enable routing (the process by which packets are forwarded or routed hop by hop across the internet from source to destination).
• Route tables can be populated statically or via protocols such as BGP, the Border Gateway Protocol.
• When an ISP router forwards a packet to another router, it does so at Layer 2, wrapping the packet in a frame.
• Address Resolution Protocol is the protocol that obtains the MAC addresses involved in frame creation.
• ARP is generally used when a layer 3 packet needs to be encapsulated inside a frame (layer 2) and then the frame sent to a MAC address.
• ARP gives you the MAC address for a given IP address.
• Routers are determined by the router table
• Each router has to have at least 1 router table
• Each roter is a colletion of routes
• Routers check destination field and will have a Next Hop or Target field.

Layer 3 - IP routing

• R1: Router One
• R2: Router Two
• D: Device
• Routers are not needed for the same local network
• Subnet Mask and destination IP has to confirm if the devices are on the same network
• ARP is needed to obtain the appropriate MAC address
• When communicating with a remote network
• Create a packet (P2), laptop address as destination
• A normal network will often drop a random packet
• The router is there to decide how to route the packet to another destination (a router in another network)

Layer 3 - Network

• Layer 3
• IP Addresses IPv4/v6 - cross network addressing
• Address resolution Protocol to find MAC Address
• Used to forward the packet
• Use routing tables
• Moves packets from source to destination through encapsulating on Layer 2
• Communications on internet
• No method for channels, simply source to destination IP
• Can be delivered out of order
• All of this amounts to device-to-device communication over the internet
• Summary • Media Access Control for device to all devices and device to device communications • Cross network addressing for IPs • Address Resolution Protocol to find MAC Address • Routes define where a packet is needed • Layer 3 works by using a router which encapsulates in Layer 2 frame all along the way from source to destination
• With this a server can communicate and so packets can move