Lakhasly

OSI Network Layer

Note for Instructors These presentations are the result of a collaboration among the instructors at St. Clair College in Windsor, Ontario.Routing Protocols Routing Information Protocol (RIP) Enhanced Interior Gateway Protocol (EIGRP) Open Shortest Path First (OSPF)Address Types Two address types: MAC address: Physical address of the host Burned in to the NIC Layer 2 address Network Address: Logical address of the host Assigned by network administrator Layer 3 address

Each Host Has Two Addresses Physical (MAC): The physical address uniquely identifies the host from all other hosts on all other networks at Layer 2.Characteristics: Connectionless "Best Effort" Delivery (Unreliable) Media Independent

Connectionless "Best Effort" Delivery (Unreliable) Unreliable means simply that IP does not have the capability to manage and recover from undelivered or corrupt packets.Replacing the switch in the diagram with a router, creates two separate IP sub-networks and two broadcast domains.Information is learned in two ways: Manual configuration of the information (Static) Information received from another router (Dynamic)

Static Routing Manually configured.An IP Address is like a telephone number:

519-972-2727 519 - Network Portion 972-2727 - Host Portion

519 - Windsor area 972-2727 - St. Clair College

Dividing Networks from Networks An IP Version 4 address has two parts: Network number Host number

The network portion of the address is the same for all hosts on the network.Routing Processes: How Routes Are Learned Routing requires that every hop, or router, along the path to a packet's destination have a route to forward the packet.Since protocols at other layers can manage reliability, IP is allowed to function very efficiently at the Network Layer.Fragmentation: At times, an intermediary device (router) will need to split up a packet when forwarding it from one media to a media with a smaller MTU.Dividing Networks from Networks The IP Version 4 Address contains elements that uniquely identify both the network and host.192.168.1.2 / 24 This method indicates Classless Routing or Classless Interdomain Routing (CIDR).192.168.1.2 / 24 This method indicates Classless Routing or Classless Interdomain Routing (CIDR).Destination Network - Routing Table Entries The hierarchical nature of Layer 3 addressing means that...The default route in a routing table performs much the same function as a default gateway in a PC. If a route for a packet cannot be found in the routing table, and a default route is present, that route will be used to forward the packet.If anyone finds any errors or omissions, please let me know at: [email protected] from Host to Host Network Layer Addresses packets with an IP Address.IPV4: Example Network Layer Protocol Internet Protocol Version 4 (IPV4) is the most widely used version of IP. Only Layer 3 protocol used on the Internet.Operates independent of the layers that handle the physical medium that carries the packet.Optical Fiber: ATM MTU = 17,966 bytes.IP Addressing - The Subnet Mask There are two methods of expressing a subnet mask.Physical (MAC): The physical address uniquely identifies the host from all other hosts on all other networks at Layer 2.Routers base their decisions on the NETWORK PORTION of the IP address when determining the best path for the packet.Dynamic Routing Routing information is exchanged among the routers using a routing protocol.Is concerned with the size of the packet or Maximum Transmission Unit (MTU).The MTU is established as part of the communication between the Data Link and Network Layers.Dividing Hosts into Groups Why Separate Hosts into Networks?

OSI Network Layer

Note for Instructors
These presentations are the result of a collaboration among the instructors at St. Clair College in Windsor, Ontario.
Thanks must go out to Rick Graziani of Cabrillo College. His material and additional information was used as a reference in their creation.
If anyone finds any errors or omissions, please let me know at:
[email protected].

Communication from Host to Host
Network
Layer
Addresses packets with an IP Address.
Encapsulates the packet.
Routes the packet to the destination.
Decapsulates the packet.

IPV4: Example Network Layer Protocol
Internet Protocol Version 4 (IPV4) is the most widely used version of IP.
Only Layer 3 protocol used on the Internet.
Focus of this course.
Internet Protocol Version 6 (IPV6) is developed and slowly being implemented. (More in CCNA-4)
Will eventually replace IPV4.
Different characteristics than IPV4.
Characteristics:
Connectionless
“Best Effort” Delivery (Unreliable)
Media Independent

Connectionless
“Best Effort” Delivery (Unreliable)
Unreliable means simply that IP does not have the capability to manage and recover from undelivered or corrupt packets.
Since protocols at other layers can manage reliability, IP is allowed to function very efficiently at the Network Layer.

Media Independent
Not concerned with the physical medium.
Operates independent of the layers that handle the physical medium that carries the packet.
Is concerned with the size of the packet or Maximum
Transmission
Unit (MTU).

The MTU is established as part of the communication between the Data Link and Network Layers.
Fragmentation:
At times, an intermediary device (router) will need to split up a packet when forwarding it from one media to a media with a smaller MTU.

Copper Ethernet: MTU = 1,518 bytes.
Copper Serial: Frame Relay MTU = 512 bytes.
Optical Fiber: ATM MTU = 17,966 bytes.
Wireless: 802.11 MTU = 2272 bytes.

Dividing Hosts into Groups
Why Separate Hosts into Networks?
Performance
Security
Address Management

Performance
Large numbers of hosts on a single
network:
Actual Data
Overhead
A big part of the overhead is broadcasts.
In this context, each network is called a broadcast domain.
Switches forward broadcasts to each device connected to a switch port.
If we can reduce broadcast overhead, it would improve performance on the network.
Routers block broadcasts unless specifically configured to
forward them.
Replacing the switch in the diagram with a router, creates two separate IP sub-networks and two broadcast domains.
Broadcasts are now contained within each network.

Dividing Networks from Networks
The IP Version 4 Address contains elements that uniquely identify both the network and host.
An IP Address is like a telephone number:

519-972-2727
519 – Network Portion
972-2727 – Host Portion

519 – Windsor area
972-2727 - St. Clair College

Dividing Networks from Networks
An IP Version 4 address has two parts:
Network number
Host number

The network portion of the address is the same for all hosts on the network.
Each device is identified by a unique host portion.

This hierarchy means that routers only need to know the network portion – not the address of each individual host.

There is a direct relationship, bit for bit, between the IP Address and it's associated subnet mask.
Any subnet mask bit that is a 1 means that the associated address bit belongs to the network number.
Any subnet mask bit that is a 0 means that the associated address bit belongs to the host number.

IP Addressing – The Subnet Mask
There are two methods of expressing a subnet mask.

The traditional method is to use the decimal value of the 1 bits that apply to the network.
192.168.1.2 255.255.255.0
This method is used for Classful Routing .

The new method is known as IP Prefix or CIDR.
Simply follow the IP address with a slash (/) and the number of bits that make up the network portion.
The remainder of the 32 bits are for the host number.
192.168.1.2 / 24
This method indicates Classless Routing or Classless Interdomain Routing (CIDR).

There are two methods of expressing a subnet mask.
The traditional method is to use the decimal value of the 1 bits that apply to the network.
192.168.1.2 255.255.255.0
This method is used for Classful Routing .

The new method is known as IP Prefix or CIDR.
Simply follow the IP address with a slash (/) and the number of bits that make up the network portion.
The remainder of the 32 bits are for the host number.
192.168.1.2 / 24
This method indicates Classless Routing or Classless Interdomain Routing (CIDR).

Address Types
Two address types:
MAC address:
Physical address of the host
Burned in to the NIC
Layer 2 address
Network Address:
Logical address of the host
Assigned by network administrator
Layer 3 address

Each Host Has Two Addresses
Physical (MAC):
The physical address uniquely identifies the host from all other hosts on all other networks at Layer 2.
This is the address that is absolutely necessary to get the information into the host. The IP address by itself won't accomplish that.

Physical (MAC):
The physical address uniquely identifies the host from all other hosts on all other networks at Layer 2.
This is the address that is absolutely necessary to get the information into the host. The IP address by itself won't accomplish that.

Logical (IP):
The logical address uniquely identifies the host and the network to which it belongs at Layer 3.
Routers base their decisions on the NETWORK PORTION of the IP address when determining the best path for the packet.

Destination Network - Routing Table Entries
The hierarchical nature of Layer 3 addressing means that…
One route entry could refer to a large general network.
Another entry could refer to a subnet of that same network.
When forwarding a packet, the router will select the most specific route.
The default route in a routing table performs much the same function as a default gateway in a PC.
If a route for a packet cannot be found in the routing table, and a default route is present, that route will be used to forward the packet.

Routing Processes: How Routes Are Learned
Routing requires that every hop, or router, along the
path to a packet's destination have a route to forward the packet.
The routing table contains the information to make packet forwarding decisions.
Information is learned in two ways:
Manual configuration of the information (Static)
Information received from another router (Dynamic)

Static Routing
Manually configured.
Must know network structure.
Every router between each source and destination must have routes.
Changes to the topology require static route changes.

Dynamic Routing
Routing information is exchanged among the routers using a routing protocol.
Route always up to date with little administration but creates overhead.

Routing Protocols
Routing Information Protocol (RIP)
Enhanced Interior Gateway Protocol (EIGRP)
Open Shortest Path First (OSPF)