Download Network Layer (Part IV) - SI-35-02

Network Layer (Part IV)
Overview
• A router is a type of internetworking device
that passes data packets between networks
based on Layer 3 addresses.
• A router has the ability to make intelligent
decisions regarding the best path for delivery
of data on the network.
• In this chapter, you will learn how routers use
a Layer 3 addressing scheme to make
forwarding decisions.
Overview
• In addition, you will learn how devices on local-area networks
(LANs) use Address Resolution Protocol (ARP) before forwarding
data to a destination.
• You will learn what happens when a device on one network does
not know the MAC address of a device on another network.
• You will learn that Reverse Address Resolution Protocol (RARP) is
the protocol a device uses when it does not know its own IP
address.
• Lastly, you will learn the difference between routing and routed
protocols and how routers track distance between locations.
• You will also learn about distance-vector, link-state, and hybrid
routing approaches and how each resolves common routing
problems.
Layer 3 Devices : Routers
• In networking, there are two addressing schemes: one
uses the MAC address, a data link (Layer 2) address;
the other uses an address located at the network layer
(Layer 3) of the OSI model.
• An example of a Layer 3 address is an IP address. A
router is a type of internetworking device that passes
data packets between networks, based on Layer 3
addresses.
• A router has the ability to make intelligent decisions
regarding the best path for delivery of data on the
network
Layer 3 Devices : Routers
Layer 3 Devices : Routers
Layer 3 Devices : Layer 3 addresses
• Bridges and switches use physical, or MAC
addresses, to make data forwarding decisions.
• Routers use a Layer 3 addressing scheme to make
forwarding decisions.
• They use IP, or logical addresses, rather than MAC
addresses.
• Because IP addresses are implemented in
software, and refer to the network on which a
device is located, sometimes these Layer 3
addresses are referred to as protocol addresses,
or network addresses
Layer 3 Devices : Layer 3 addresses
• Physical, or MAC addresses, are usually assigned by the NIC
manufacturer and are hard-coded into the NIC.
• The network administrator usually assigns IP addresses.
• In fact, it is not unusual for a network administrator to group
devices together in the IP addressing scheme, according to their
geographical location, department, or floor within a building.
• Because they are implemented in software, IP addresses are
fairly easy to change.
• Finally, bridges and switches are primarily used to connect
segments of a network.
• Routers are used to connect separate networks and to access
the worldwide Internet. They do this by providing end-to-end
routing
Layer 3 Devices : Layer 3 addresses
Layer 3 Devices : Layer 3 addresses
Layer 3 Devices : Unique network
numbers
• Routers connect two or more networks, each
of which must have a unique network number
in order for routing to be successful.
• The unique network number is incorporated
into the IP address that is assigned to each
device attached to that network.
Layer 3 Devices : Unique network
numbers
• Example:
A network has a unique network number - A.
It has four devices attached to it. The IP
addresses of the devices are A2, A3, A4, and
A5. Since the interface where the router
connects to a network is considered to be part
of that network, the interface where the
router connects to network A has an IP
address of A1
Layer 3 Devices : Unique network
numbers
• Example:
Another network, with a unique network
number - B - has four devices attached to it.
This network is also attached to the same
router, but at a different interface. The IP
addresses of the devices on this second
network are B1, B2, B3, and B4. The IP address
of the router's second interface is B5.
Layer 3 Devices : Unique network
numbers
• Example:
You want to send data from one network to another. The
source network is A; the destination network is B; and a
router is connected to networks A, B, C, and D. When data
(frames), coming from network A, reaches the router, the
router performs the following functions:
– It strips off the data link header, carried by the frame. (The data
link header contains the MAC addresses of the source and
destination.)
– It examines the network layer address to determine the
destination network.
– It consults its routing tables to determine which of its interfaces
it will use to send the data, in order for it to reach its destination
network.
Layer 3 Devices : Unique network
numbers
• In the example, the router determines that it
should send the data from network A to
network B, from its interface, with address B5.
Before actually sending the data out interface
B5, the router would encapsulate the data in
the appropriate data link frame.
Layer 3 Devices : Unique network
numbers
Layer 3 Devices : Unique network
numbers
Layer 3 Devices : Unique network
numbers
Layer 3 Devices : Router
interface/port
• A router’s attachment to a network is called
an interface; it may also be referred to as a
port.
• In IP routing, each interface must have a
separate, unique network (or subnetwork)
address.
Layer 3 Devices : Router
interface/port
Network-to-Network Communications :
Methods for assigning an IP address
• After you have determined the addressing
scheme for a network, you must choose the
method for assigning addresses to hosts.
• There are essentially two methods for
assigning IP addresses - static addressing and
dynamic addressing.
• Regardless of which addressing scheme you
use, no two interfaces can have the same IP
address.
Network-to-Network Communications :
Methods for assigning an IP address
Static Addressing
If you assign IP addresses statically, you must go to each
individual device and configure it with an IP address.
• This method requires you to keep very meticulous records,
because problems can occur on the network if you use
duplicate IP addresses.
• Some operating systems, such as Windows 95 and
Windows NT, send an ARP request to check for a duplicate
IP address when they attempt to initialize TCP/IP.
• If they discover a duplicate, the operating systems will not
initialize TCP/IP and will generate an error message.
• Record keeping is important too, because not all operating
systems identify duplicate IP addresses.
Network-to-Network Communications :
Methods for assigning an IP address
• Dynamic
Addressing
There are a few different methods that you can use to
assign IP addresses dynamically. Examples of these are:
• Reverse Address Resolution Protocol (RARP)
Reverse address resolution protocol (RARP) binds MAC
addresses to IP addresses. This binding allows some
network devices to encapsulate data before sending
them out on the network. A network device such as a
diskless workstation might know its MAC address, but
not its IP address. Devices using RARP require that a
RARP server be present on the network to answer
RARP requests.
Network-to-Network Communications :
Methods for assigning an IP address
• Let's look at an example where a source device wants to
send data to another device.
• In our example the source knows its own MAC address, but
is unable to locate its own IP address in its ARP table.
• In order for the destination device to retrieve the data, pass
it to higher layers of the OSI model, and respond to the
originating device, the source must include both its MAC
address and IP address.
• Therefore, the source initiates a process called a RARP
request, which helps it detect its own IP address. The
device builds a RARP request packet and sends it out on the
network. To ensure that all devices see the RARP request
on the network, it uses a broadcast IP address.
Network-to-Network Communications :
Methods for assigning an IP address
• RARP uses the same packet format as ARP. But in a
RARP request, the MAC headers, IP headers, and
"operation code" are different from an ARP request.
• The RARP packet format contains places for MAC
addresses of both destination and source.
• The source IP address field is empty. The broadcast
goes to all devices on the network; therefore the
destination IP address will be set to all binary 1s.
• Workstations running RARP have codes in ROM that
direct them to start the RARP process, and locate the
RARP server.
Network-to-Network Communications :
Methods for assigning an IP address
• BOOTstrap Protocol (BOOTP)
A device uses BOOTstrap protocol (BOOTP) when
it starts up, to obtain an IP address.
• BOOTP uses UDP to carry messages; the UDP
message is encapsulated in an IP datagram.
• A computer uses BOOTP to send a broadcast IP
datagram (using a destination IP address of all 1s
- 255.255.255.255).
• A BOOTP server receives the broadcast and then
sends a broadcast. The client receives a datagram
and checks the MAC address.
Network-to-Network Communications :
Methods for assigning an IP address
• If it finds its own MAC address in the destination address
field, then it takes the IP address in that datagram.
• Like RARP, BOOTP operates in a client-server environment,
and only requires a single packet exchange.
• However, unlike RARP, which only sends back a 4 octet IP
address, BOOTP datagrams can include the IP address, the
address of a router (default gateway), the address of a
server, and a vendor-specific field.
• One of the problems with BOOTP is that it was not
designed to provide dynamic address assignment.
• With BOOTP you create a configuration file that specifies
the parameters for each device.
Network-to-Network Communications :
Methods for assigning an IP address
• Dynamic Host Configuration Protocol (DHCP)
Dynamic host configuration protocol (DHCP) has been
proposed as a successor to BOOTP.
• Unlike BOOTP, DHCP allows a host to obtain an IP address
quickly and dynamically.
• All that is required using DHCP is a defined range of IP
addresses on a DHCP server. As hosts come online they
contact the DHCP server and request an address.
• The DHCP server chooses an address and allocates it to that
host.
• With DHCP, the entire computer’s configuration can be
obtained in one message (e.g. along with the IP address,
the server can also send a subnet mask).
Network-to-Network Communications :
Methods for assigning an IP address
Network-to-Network Communications :
Methods for assigning an IP address
Network-to-Network Communications : DHCP
initialization sequence
• When a DHCP client boots, it enters an initialize state. It sends
DHCPDISCOVER broadcast messages, which are UDP packets
with the port number set to the BOOTP port.
• After sending the DHCPDISCOVER packets, the client moves
into the select state and collects DHCPOFFER responses from
DHCP server.
• The client then selects the first response it receives and
negotiates lease time (the length of time it can keep the
address without renewing it) with the DHCP server by sending
a DHCPREQUEST packet.
• The DHCP server acknowledges a client request with a
DHCPACK packet.
• The client can now enter the bound state and begin using the
address.
Network-to-Network Communications : DHCP
initialization sequence
Network-to-Network Communications :
IP key components
• In order for devices to communicate, the sending
devices need both, the IP addresses and the MAC
addresses, of the destination devices.
• When they try to communicate with devices
whose IP addresses they know, they must
determine the MAC addresses.
• The TCP/IP suite has a protocol, called ARP, that
can automatically obtain the MAC address.
• ARP enables a computer to find the MAC address
of the computer that is associated with an IP
address.
Network-to-Network Communications :
IP key components
• Note: The basic unit of data transfer in IP is the IP
packet.
• Packet processing occurs in software, which
means that content and format are not hardware
dependent.
• A packet is divided into two major components:
the header, which includes source and
destination addresses; and the data.
• Other types of protocols have their own formats.
The IP packet is unique to IP.
Network-to-Network Communications :
IP key components
• Note: Another major component of IP is Internet
Control Message Protocol (ICMP).
• This protocol is used by a device to report a
problem to the sender of a message.
• For example, if a router receives a packet that it
cannot deliver, it sends a message back to the
sender of the packet.
• One of the many features of ICMP is echorequest/echo-reply, which is a component that
tests whether a packet can reach a destination by
pinging the destination.
Network-to-Network Communications :
IP key components
Network-to-Network Communications : Function of
the address resolution protocol (ARP)
• Layer 3 protocols determine whether data passes
beyond the network layer to higher levels of the OSI
model.
• A data packet must contain both, a destination MAC
address and a destination IP address.
• If it lacks one or the other, the data will not pass from
Layer 3 to the upper layers.
• In this way, MAC addresses and IP addresses act as
checks and balances for each other.
• After devices determine the IP addresses of the
destination devices, they can add the destination MAC
addresses to the data packets.
Network-to-Network Communications : Function of
the address resolution protocol (ARP)
• There are a variety of ways that devices can
determine the MAC addresses they need to
add to the encapsulated data.
• Some keep tables that contain all the MAC
addresses and IP addresses of other devices
that are connected to the same LAN.
• They are called Address Resolution Protocol
(ARP) tables, and they map IP addresses to the
corresponding MAC addresses.
Network-to-Network Communications : Function of
the address resolution protocol (ARP)
• ARP tables are sections of RAM memory, in which
the cached memory is maintained automatically
on each of the devices.
• It is a rare occasion when you must make an ARP
table entry manually.
• Each computer on a network maintains its own
ARP table.
• Whenever a network device wants to send data
across a network, it uses information provided by
its ARP table.
Network-to-Network Communications : Function of
the address resolution protocol (ARP)
• When a source determines the IP address for a
destination, the source consults its ARP table in
order to locate the MAC address for the destination.
• If the source locates an entry in its table (destination
IP address to destination MAC address), it binds, or
associates, the IP address to the MAC address and
uses it to encapsulate the data.
• The data packet is then sent out over the networking
media to be picked up by the destination.
Network-to-Network Communications : Function of
the address resolution protocol (ARP)
Network-to-Network Communications : Function of
the address resolution protocol (ARP)