Network Layer & Routing
Dr John C. Murray Principal Lecturer
Network Layer
Data Link Layer
Data Link Layer
• Takes packets from network layer above – Creates Frames to send to Physical Layer
• Error Detection and Correction
– Also responsible for detecting and correcting errors in receiving data.
Data Link Layer
• Multiple access protocols
– Frequency Division Multiple Access (FDMA)
– Time Division Multiple Access (TDMA)
Data Link Layer
• Protocols
– ARP
• Assigning IPv4 to MAC Addresses
– Token Ring
• Assigning the permission token
– Point-to-Point Protocol • Between two nodes
– Plus many, many others
Network Layer
• End to end delivery of data
– Between two computers
– This is not ‘devices’
• But an actual entire route
– Addressing – Routing
Network Layer
• So, the network layer is responsible for packet forwarding including routing through intermediate routers
• Whereas the data link layer is responsible for media access control, flow control and error checking.
Postal Service Analogy
Post Codes and Addresses
Routing Data
Network Layer
• The Network Layer defines three main features:
– Routing – defines how devices (typically routers) forward packets to their final destination
– Logical Addressing – defines how each device can have an address that can be used by the routing process
– Path determination – refers to the work done by routing protocol by which all possible routes are learned, but the best route is chosen for use.
Network Layer
• Routing Protocol
– Aids routers by dynamically learning about the groups of addresses in the network which allows for forwarding and routing of packets.
• Other utilities
– Network layer also relies on:
– DNS – Domain Name System
– DHCP – Dynamic Host Configuration Protocol – ARP – Address Resolution Protocol
TCP/IP
• IP – Purpose is to deliver packets with as little per-packet effort as possible
– No error checking
• TCP – Performs the useful networking functions
– Error recovery
– Resending lost packets
Forwarding
• Focuses on the end-to-end logic of data
• Here PC1 is sending to PC2 via interconnected routers
• The logic used by each device varies
Routing
• The sending computer does not know much about the network
– Only very local information • E.g. the connected Router
• Post office analogy
– Knowing how to get to the local post office
Routing
• How is all this information managed? – Routers local knowledge
– Routing Tables
– Built Automatically
• Hop-by-Hop
– Lists the next device in the path to the destination
– Fundamental in the IP / Network Layer
Constructing a Routing Table
Routers
• Act as a default gateway
• Move data between networks
• Restrict network broadcasts
• Switches create a network. Routers connect networks. A router links computers to the internet, so users can share the connection.
• A router acts as a dispatcher, choosing the best path for information to travel so it’s received.
Routers – Default Gateway
• When a computer wants to talk to a computer on another network, it does so by sending data to the default gateway (local router)
• Data is passed between routers until the data reaches the destination network.
Routing Data
IPv4 Addressing
• Format of the IPv4 address
– Left field(s) identify the network
– Right field(s) identify the host machine
IP Addressing – Address Ranges
http://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
Network Classes
Host Allocation
IP Addressing
• The class A address is thus useful where there are a small number of networks with a large number of hosts connected to them.
• Class C is useful where there are many networks with a relatively small number of hosts connected to each network.
• Class B gives a good compromise between of networks and connected hosts.
Classless Network
• If we have more than 256 machines – What can we do?
– Get a Class B network
– What if we only have 300 machines?
– Class B can handle 65,536 machines!
• Change the subnet to a classless value
https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing
Questions