© 2007 Cisco Systems, Inc. All rights reserved.Cisco Public 1 Version 4.0 OSI Network Layer Network Fundamentals – Chapter 5

2 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Objectives  Identify the role of the Network Layer, as it describes communication from one end device to another end device.  Examine the most common Network Layer protocol, Internet Protocol (IP), and its features for providing connectionless and best-effort service.  Understand the principles used to guide the division or grouping of devices into networks.  Understand the hierarchical addressing of devices and how this allows communication between networks.  Understand the fundamentals of routes, next hop addresses and packet forwarding to a destination network.

3 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Protocols and Internet Protocol (IP)  Layer 3 of OSI  Receives segments or PDUs from TL  4 tasks: Addressing packets with an IP address Encapsulation Routing Decapsulation

4 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Protocols and Internet Protocol (IP)

5 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Tasks Addressing packets with an IP address Each sending and receiving device must have unique IP address Device with IP address = hosts Sending host = source IP address Receiving host = destination IP address

6 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Tasks Encapsulation IP header – source and destination IP addresses Process of adding information = encapsulation Encapsulated PDU = packet

7 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Tasks Routing Routers =device that connect networks Routers – understand packets and calculating best path for packets Routing = process perform by routers : receive packets, analyzing dest add info, select a path and forwarding packets to the next router Each route to next device = hop Decapsulation Process of removing encapsulation data Actually encap and decap happened at all layers of OSI model

8 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Network Layer Protocols

9 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Most widely used  Basic characteristics:

10 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Connectionless No established connection IP simply sends packets without informing receiver Requires less data to perform required tasks – uses much less processing power and bandwidth = overhead

11 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4

12 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Best Effort TCP is reliable IP is unreliable IP makes a ‘best effort’ to deliver packets TCP can be relied on delivery problems TCP/IP – TL & NL

13 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Best Effort

14 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Media independent IP is not concerned with physical medium that carries packet Internetwork communication – multimedia journey ex. wireless, ethernet cable, fiber optic

15 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  Media independent

16 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4  IPv4 encapsulates or packages the TL segment or datagram as packets

17 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4 Packet Header  IP Source Address 32 bits  IP Destination Address 32 bits  Time to Live (TTL) 8 bits Max hops the packet can take before considered lost/undeliverable Each router decrements TTL field by at leased 1 If TTL reaches 0 – packet will be dropped

18 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4 Packet Header  Type of Service (ToS) 8 bits Describes level of throughput Ex – voice data precede streaming music Quality of Service - QOS  Protocol 8 bits Indicate upper layer protocol TCP, UDP or ICMP  Flag and Fragment Offset Packet fragmented – small MTU Used to reconstruct the packets

19 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4 Packet Header  Version IPv4 or IPv6  Internet Header Length (IHL) How long the header - Options may caused different lengths  Packet Length Total length of datagram including the header Min 20 bytes, max 65,535 bytes  Identification Help reassemble any fragments

20 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public IPv4 Packet Header  Header Checksum Indicate length of header Checked by each router If invalid, packet assumed to be corrupted and is dropped Relation to TTL?  Options Special routing services  Padding Fill bits when header data does not end on 32 bits boundary

21 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public TBC - Wednesday

22 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Networks are communities of computers and other hosts  Like human communities  Small town Easy to find and communicate, not need large roads & expensive traffic signals, not many services, trust each other and considered safer  Large town Ex..address  Same to computer communities  More planning to address the network so it can be managed efficiently

23 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Grouping devices into sub-networks

24 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Large  Computer networks can be separated into internetworks  Departments and groups share computers and servers into common subnetwork or subnet  Geographically, Specific Purpose, Ownership

25 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Geographically

26 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Specific Purpose Different user – different reasons, different tools, different requirements

27 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Grouping Devices into Networks and Hierarchical Addressing  Ownership Main concern – security

28 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Performance degradation  Security issues  Address management

29 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Performance Hosts can be chatty devices Broadcast news about themselves Broadcast = message sent from one host to all other hosts on the network Share own information and request information about other hosts More broadcast = more bandwidth consumed Broadcast domain

30 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Performance

31 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Security Isolating and shielding devices from public access Better protection Local network manager can more easily control outside access to the smaller network Router or firewall at the perimeter of the network Configured – known & trusted data/user to access network

32 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Security

33 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Address Management Gateway router – send/receive messages beyond the network

34 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Why Separate Hosts into Networks?  Address Management Hierarchical Addressing

35 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Dividing Networks from Networks  IPv4 address = 32 bits  Two parts Network = 24 bits - postcode Host = 8 bits - destination

36 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Intermediary gateway device allowing devices to communicate across sub-divided networks  A host has a default gateway address defined  Ipconfig command

37 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  IP packet traverses unchanged via routers from sub network to sub-network

38 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Gateway needed to send packet out of the network  Routers add routes for the connected network to their routing table  Routing table stores information about connected and remote networks  When configured with IP and subnet mask, the interface becomes part of the network  The routing table includes that network as directly connected network.

39 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding

40 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  3 mains features of routes in routing table Destination network Next-hop Metric Hop Count Delay

41 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Destination network in routing table entry represents a range of host addresses or network and host addresses

42 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Next Hop – where the packet goes next  Next hop is the address of the device that will process the packet next

43 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Steps of IP packets as they are routed through several gateways from devices on one sub network to devices on other sub networks

44 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Static Routing Manually configured route information on the router Low router processing overhead, High administrative cost  Dynamic Routing Routers can learn about routes automatically from other routers High router processing overhead, Little administrative cost  Routing protocols Are the set of rules by which router dynamically share their routing information -Routing Information Protocols (RIP) -Enhance interior Gateway Protocol (EIGRP) - Open Shortest Path First (OSPF)

45 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Static Routing

46 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Fundamentals of Routes, Next Hop Addresses and Packet Forwarding  Dynamic Routing

47 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public Summary

48 © 2007 Cisco Systems, Inc. All rights reserved.Cisco Public